DNA molecules coding for debranching enzymes derived from plants

Abstract

The invention describes DNA molecules which code for plant proteins having the biological activity of a debranching enzyme. Furthermore described are transgenic plant cells and plants having reduced or increased debranching enzyme activity, as well as modified starch isolatable from said cells and plants.

Description

[0001] The present invention relates to DNA molecules coding for proteins from plants having the enzymatic activity of a debranching enzyme (R enzyme). The invention furthermore relates to a process for modifying the branching degree of amylopectin synthesized in plants, and to plants and plant cells in which an amylopectin having a modified branching degree is synthesized due to the expression of an additional debranching enzyme activity or the inhibition of an endogenous debranching enzyme activity, as well as to the starch obtainable from said plant cells and plants.

[0002] Starch plays an important role both as storage substance in a variety of plants and as reproductive, commercially useful raw material and is gaining significance. For the industrial application of starch it is necessary that the starch meets the requirements of the manufacturing industry in terms of its structure, form and/or other physico-chemical parameters. For the starch to be useful in as many fields of application as possible it is furthermore necessary that it is obtainable in as many forms as possible.

[0003] While the polysaccharide starch is composed of chemically uniform components, the glucose molecules, it is a complex mixture of different molecule forms that exhibit differences as regards their polymerization degree and the presence of branches. One distinguishes the amylose starch, an essentially unbranched polymer of α-1,4 glycosidically linked glucose molecules from the amylopectin starch, a branched polymer, the branches of which are the result of additional α-1,6 glycosidic bonds.

[0004] In plants typically used for starch production, such as maize or potato, both starch forms are present in a ratio of about 25 parts of amylose to 75 parts of amylopectin. In addition to amylopectin, maize, for example, exhibits another branched polysaccharide, the so-called phytoglycogen which differs from the amylopectin by a higher branching degree and a differing solubility (see, e.g., Lee et al., Arch. Biochem. Biophys. 143 (1971), 365-374; Pan and Nelson, Plant Physiol. 74 (1984), 324-328). In the present application the term amylopectin is intended to comprise phytoglycogen.

[0005] With a view to the uniformity of the basic compound starch for its industrial application starch-producing plants are required that contain, e.g., either only the component amylopectin or only the component amylose. For other applications plants are required that synthesize forms of amylopectin of different degrees of branching.

[0006] Such plants can be generated, e.g., by breeding or mutagenesis techniques. It is known of certain plant species, e.g., maize, that mutagenesis can be used to generate varieties producing only amylopectin. For potato, a genotype was generated by chemical mutagenesis for a haploid line that does not produce amylose (Hovenkamp-Hermelink, Theor. Appl. Genet. 75 (1987), 217-221). Haploid lines, however, or the homozygous diploid or tetraploid lines derived thereof are not useful for agricultural purposes. Mutagenesis techniques, however, cannot be applied to the tetraploid lines that are interesting for agriculture since due to the presence of four different genotypes inactivation of all copies of a gene is not technically feasible. Therefore, in the case of potato, one must fall back on other techniques, e.g., the specific genetically engineered modification of plants.

[0007] For example it is known from Visser et al. (Mol. Gen. Genet. 225 (1991), 289) and WO 92/11376 that varieties can be generated by anti-sense inhibition of the gene for the starch granule-bound starch synthase in potato that synthesize substantially pure amylopectin starch. WO 92/14827 discloses DNA sequences coding for a branching enzyme (Q enzyme) that introduces α-1,6 branches into amylopectin starch. With these DNA sequences it should be possible to generate transgenic plants which exhibit a modified amylose/amylopectin ratio of the starch.

[0008] In order to furthermore specifically modify the branching degree of starch synthesized in plants by using genetic engineering it is still necessary to identify DNA sequences coding for enzymes that are involved in starch metabolism, particularly in the branching of starch molecules.

[0009] In addition to the Q enzymes that are capable of introducing branches into starch molecules plants comprise enzymes that are capable of dissolving branching. These enzymes are referred to as debranching enzymes and are divided into three groups in terms of their substrate specificity:

[0010] (a) Pullulanases that use amylopectin as substrate in addition to pullulan can be found in microorganisms such as Klebsiella and in plants. In plants these enzymes are often referred to as R enzymes.

[0011] (b) Isoamylases that do not use pullulan but glycogen and amylopectin as substrate can likewise be found in microorganisms and plants. Isoamylases have been described, e.g., for maize (Manners and Rowe, Carbohydr. Res. 9 (1969), 107) and potato (Ishizaki et al., Agric. Biol. Chem. 47 (1983), 771-779).

[0012] (c) Amylo-1,6-glucosidases have been described for mammals and yeasts and use limit dextrins as substrates.

[0013] Li et al. (Plant Physiol. 98 (1992), 1277-1284) succeeded in detecting in sugar beet only one debranching enzyme of the pullulanase type in addition to five endoamylases and two exoamylases. Having a size of about 100 kD and a pH optimum of 5.5, this enzyme is localized in the chloroplasts. For spinach, too, a debranching enzyme has been described that uses pullulan as substrate. Both the debranching enzyme of spinach and that of sugar beet possess an activity that is lower by a factor of 5 when reacting it with amylopectin as substrate instead of pullulan as substrate (Ludwig et al., Plant Physiol. 74 (1984), 856-861; Li et al., Plant Physiol. 98 (1992), 1277-1284).

[0014] The activity of a debranching enzyme was examined by Hobson et al. (J. Chem. Soc. (1951), 1451) for potato which is a starch-storing cultivated plan+ that is important from the agricultural point of view. They succeeded in proving that the corresponding enzyme—in contrast to the Q enzyme—does not possess chain-extending activity but merely hydrolyzes α-1,6 glycosidic bonds. However, it has not been possible so far to characterize the enzyme in more detail.

[0015] For potato processes for the purification of the debranching enzyme as well as partial peptide sequences of the purified protein have been proposed (German patent application P 43 27 165.0 and PCT/EP94/026239). In principle, it should be possible to identify DNA molecules coding for the respective proteins by means of known peptide sequences when using degenerate oligonucleotide probes. However, in practice, often the problem arose that the degree of degeneration of the probe is too high or the probes are too short to specifically identify sequences coding for the desired protein.

[0016] Despite the knowledge of the proposed peptide sequences of the debranching enzyme of potato researchers so far have not been able to isolate DNA molecules coding for debranching enzymes of plants by hybridization to degenerate oligonucleotides or by other genetic or immunological approaches such as proposed in German patent application P 43 27 165.0.

[0017] For spinach, too, for which the purification of the debranching enzyme has been described by Ludwig et al. (Plant Physiol. 74 (1984), 856-861), researchers have not been able to either determine peptide sequences or to identify DNA molecules coding for said protein.

[0018] The problem underlying the present invention is therefore to provide DNA molecules coding for proteins of plants having the enzymatic activity of a debranching enzyme and allowing to generate transgenic plant cells and plants having an increased or reduced activity of a debranching enzyme.

[0019] The problem is solved by the provision of the embodiments described in the claims.

[0020] The present invention therefore relates to DNA molecules coding for proteins of plants having the biological activity of a debranching enzyme, or a biologically active fragment thereof.

[0021] Such a DNA molecule preferably codes for a debranching enzyme of plants comprising the amino acid sequence indicated in Seq ID No. 18 or Seq ID No. 24. More preferably, such a DNA molecule comprises the nucleotide sequence indicated in Seq ID No. 17 or Seq ID No. 23, particularly the coding region thereof.

[0022] The subject matter of the invention are also DNA molecules coding for proteins of plants having the biological activity of a debranching enzyme, or biologically active fragments thereof and that hybridize to any of the DNA molecules described above.

[0023] The term “hybridization” in this context means hybridization under conventional hybridization conditions, preferably under stringent conditions such as described by, e.g. Sambrook et al. (1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). These DNA molecules that hybridize to the DNA molecules according to the present invention in principle can be derived from any plant possessing such DNA molecules. Preferably, they are derived from monocotyledonous or dicotyledonous plants, preferably from useful plants, and most preferably from starch-storing plants.

[0024] DNA molecules hybridizing to the DNA molecules of the present invention can be isolated, e.g, from genomic libraries or cDNA libraries of various plants.

[0025] Such DNA molecules from plants can be identified and isolated by using the DNA molecules of the present invention or fragments of these DNA molecules or the reverse complements of these molecules, e.g., by hybridization according to standard techniques (see, e.g., Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).

[0026] As hybridization probe, e.g., DNA molecules can be used that have exactly or substantially the same DNA sequence indicated in Seq ID No. 17 or Seq ID No. 23 or fragments of said sequence. The DNA fragments used as hybridization probes can also be synthetic DNA fragments obtained by conventional DNA synthesis techniques and the sequence of which is substantially identical to that of the DNA molecules according to the invention. Once genes hybridizing to the DNA molecules of the invention have been identified and isolated it is necessary to determine the sequence and to analyze the properties of the proteins coded for by said sequence.

[0027] The term “hybridizing DNA molecules” includes fragments, derivatives and allelic variants of the above-described DNA molecules that code for the above-described protein or a biologically active fragment thereof. Fragments are understood to be parts of DNA molecules long enough to code for the described protein or a biologically active fragment thereof. The term “derivative” means in this context that the DNA sequences of these molecules differ from the sequences of the above-described DNA molecules in one or more positions and are highly homologous to said DNA sequence. Homology is understood to refer to a sequence identity of at least 40%, particularly an identity of at least 60%, preferably more than 80% and still more preferably more than 90%. The deviations from the DNA molecules described above can be the result of deletion, substitution, insertion, addition or recombination.

[0028] Homology furthermore means that the respective DNA sequences or encoded proteins are functionally and/or structurally equivalent. The DNA molecules that are homologous to the DNA molecules described above and that are derivatives of said DNA molecules are regularly variations of said DNA molecules which represent modifications having the same biological function. They may be naturally occurring variations, such as sequences of other plant species, or mutations. These mutations may occur naturally or may be achieved by specific mutagenesis. Furthermore, these variations may be synthetically produced sequences.

[0029] The allelic variants may be naturally occurring variants as well as synthetically produced or genetically engineered variants.

[0030] The proteins encoded by the various variants of the DNA molecules of the invention share specific common characteristics, such as enzymatic activity, molecular weight, immunological reactivity, conformation, etc., as well as physical properties, such as electrophoretic mobility, chromatographic behavior, sedimentation coefficients, solubility, spectroscopic properties, stability, pH optimum, temperature optimum, etc.

[0031] Enzymatic activity of the debranching enzyme can be detected in a iodine stain test such as described in Example 5. This test is based on the finding that a protein having a starch-modifying activity can be detected by separating protein extracts, e.g., from tubers, in non-denaturing amylopectin-containing polyacrylamide gels (PAAG) and subsequently staining the gel, after incubation in a suitable buffer, with iodine. While unbranched amylose forms a blue complex with iodine, amylopectin results in a reddish-purple color.

[0032] Amylopectin-containing polyacrylamide gels giving a reddish-purple color when reacted with iodine result in a change of color up to a blue color of the gel at places where the debranching activity is localized, since the branches of the purple-staining amylopectin are digested by the debranching enzyme.

[0033] Alternatively, debranching enzyme activity can be detected by the DNSS test (see Ludwig et al., Plant Physiol. 74 (1984) , 856-861).

[0034] The present invention furthermore relates to DNA molecules the sequences of which differ from the sequences of the above-identified DNA molecules due to degeneracy of the genetic code, and which code for a protein of a plant having the biological activity of a debranching enzyme or for a biologically active fragment thereof.

[0035] According to a preferred embodiment the protein coded for by the DNA molecules according to the invention contains at least one of the peptide sequences depicted in Seq ID No. 1 to Seq ID No. 14.

[0036] According to another preferred embodiment the debranching enzymes of plants coded for by the DNA molecules of the invention can be isolated from plant protein extracts by fractionated ammonium sulfate precipitation and subsequent affinity chromatography on β-cyclodextrin.

[0037] Preferably, the DNA molecules of the invention code for proteins that exhibit a molecular weight between 50 and 150 kD in SDS gel electrophoresis, preferably between 70 and 130 kD, and most preferably between 90 and 110 kD.

[0038] In principle, the DNA molecules according to the invention can be derived from any plant organism that expresses the proteins described, preferably from taxonomically higher plants, particularly from monocotyledonous or dicotyledonous plants, preferably from plants that synthesize or store starch. Most preferred are, e.g., cereals (such as barley, rye, oat, wheat, etc.), maize, rice, pea, cassava, etc.

[0039] According to a preferred embodiment the DNA molecules of the invention are derived from plants of the family Solanaceae or from plants of the family Chenopodiaceae, preferably from Solanum tuberosum or Spinacia oleracea.

[0040] The invention furthermore relates to vectors, particularly plasmids, cosmids, viruses, bacteriophages and other vectors conventional in genetic engineering that contain the above-described DNA molecules of the invention.

[0041] According to a preferred embodiment the DNA molecules contained in the vectors are linked to regulatory DNA sequences allowing transcription and translation in procaryotic or eucaryotic cells.

[0042] According to another embodiment the invention relates to host cells, particularly procaryotic or eucaryotic cells that have been transformed with a DNA molecule or a vector described above, and cells that are derived from such host cells.

[0043] Furthermore, the present invention relates to processes for producing a protein of a plant having the biological activity of a debranching enzyme or a biologically active fragment thereof, wherein host cells according to the invention are cultivated under suitable conditions and the protein is obtained from the culture.

[0044] Another subject matter of the invention are the proteins obtainable by said process.

[0045] The invention furthermore relates to proteins of plants having the biological activity of a debranching enzyme that are coded for by the DNA molecules of the invention, except for the proteins obtained from spinach and potato that have already been described.

[0046] By providing the DNA molecules of the invention it is possible to genetically engineer plant cells such that they exhibit an increased or reduced debranching enzyme activity as compared to wild type cells. Such a modified starch is suitable for various purposes.

[0047] According to a preferred embodiment the host cells of the invention are transgenic plant cells that exhibit an increased debranching enzyme activity as compared to non-transformed cells due to the presence and expression of an additionally introduced DNA molecule of the invention.

[0048] Another subject matter of the invention are transgenic plants containing the transgenic plant cells described above.

[0049] The invention furthermore relates to the starch obtainable from the transgenic plant cells or plants. Due to the increased debranching enzyme activity the amylopectin starch synthesized by the transgenic cells or plants has properties differing from those of starch from non-transformed plants.

[0050] For example, when analyzing the viscosity of aqueous solutions of this starch upon treating they display a maximum viscosity lower than that of starch of non-transformed plants. Preferably the value of the maximum viscosity is reduced by at least 40%, particularly by at least 55% and still more preferably by at least 65% in comparison with the maximum viscosity of starch from wild type plants. Furthermore, the final viscosity of aqueous solutions of the modified starch after cooling is higher than that of wild type starch. Preferably, the final viscosity is at least 10% higher, particularly at least 30% and still more preferably at least 50% higher than that of starch form wild type plants.

[0051] Moreover, the stability of gels consisting of the modified starch is higher than that of gels of wild type starch. The force that is required to deform gels of the modified starch is greater by a factor of at least 2.5, particularly of at least 3.5 and still more preferably of at least 5.5 than that required to deform gels of wild type starch. Furthermore, the phosphate content of the modified starch is comparable to that of wild type starch.

[0052] Another object of the invention is the use of the described starch for the production of food and industrial products.

[0053] Another subject matter of the invention is propagating material of the plants of the invention, such as seeds, fruit, cuttings, tubers, root stocks, etc., with this propagating material containing transgenic plant cells described above.

[0054] The present invention furthermore relates to transgenic plant cells in which the activity of the debranching enzyme is reduced due to the inhibition of the transcription or translation of endogenous nucleic acid molecules coding for a debranching enzyme. This is preferably achieved by expressing a DNA molecule of the invention in the respective plant cells in antisense direction. Due to an antisense effect the debranching enzyme activity is reduced. Another possibility of reducing the debranching enzyme activity in plant cells is to express suitable ribozymes that specifically cleave transcripts of the DNA molecules of the invention. The production of such ribozymes using the DNA molecules of the invention is known in the art. Alternatively, the debranching enzyme activity in the plant cells may be reduced by a co-suppression effect.

[0055] The invention furthermore relates to transgenic plants containing the transgenic plant cells described above having reduced debranching enzyme activity. Another subject matter of the invention is the modified starch obtainable from the transgenic cells or plants. The amylopectin starch of the transgenic cells and plants exhibits an altered branching degree as compared to the starch of non-transformed plants due to the reduced debranching enzyme activity. Furthermore, the modified starch obtainable from the described transgenic plants may differ in several aspects form starch of wild type plants. For example, when analyzing the viscosity of aqueous solutions of this starch upon heating they display a maximum viscosity that is lower than that of starch from non-transformed plants. Preferably, the value of the maximum viscosity is reduced by at least 35%, particularly by at least 40% and still more preferably by at least 50% in comparison to the maximum viscosity of starch from wild type plants.

[0056] Starch granules of the modified starch synthesized by plants with reduced debranching enzyme activity may have a rough, chapped or even frayed surface.

[0057] Furthermore, the modified starch is characterized in that gels produced from this starch are more stable than gels of wild type starch. The force that is required to deform gels of the modified starch is greater by a factor of at least 2.3, more preferably by at least 3.8 and still more preferably by at least 6.0 than that required to deform gels of wild type starch.

[0058] The phosphate content of the modified starch is preferably higher than that of wild type starch. The increase in phosphate content depends on the degree of reduction of the debranching enzyme activity. Preferably, the phosphate content is at least 15%, more preferably at least 25% and still more preferably at least 60% higher than that of wild type starch.

[0059] Still another object of the invention is the use of the described starch for the production of food or industrial products.

[0060] The invention also relates to propagating material of the above-described transgenic plants, such as seeds, fruit, cuttings, tubers, root stocks, etc., with the propagating material containing above-described transgenic plant cells.

[0061] Transgenic plant cells that due to the additional expression of a debranching enzyme generate an amylopectin starch having an altered branching degree as compared to the amylopectin starch synthesized by wild type plants are obtainable, e.g., by a process comprising the following steps:

[0062] (a) Producing an expression cassette comprising the following DNA sequences:

[0063] (i) a promoter allowing transcription in plant cells;

[0064] (ii) at least one DNA sequence coding for a protein having the enzymatic activity of a debranching enzyme and being fused to the 3′ end of the promoter in sense orientation; and

[0065] (iii) optionally a termination signal for the transcription termination and the addition of a poly-A tail to the transcript formed that is coupled to the 3′ end of the coding region; and

[0066] (b) transformation of plant cells with the expression cassette produced in step (a).

[0067] Transgenic plant cells that due to the reduction of the activity of a debranching enzyme generate an amylopectin starch having an altered branching degree as compared to the amylopectin starch synthesized by wild type plants are obtainable, e.g., by a process comprising the following steps:

[0068] (a) Producing an expression cassette comprising the following DNA sequences:

[0069] (i) a promoter allowing transcription in plant cells;

[0070] (ii) at least one DNA sequence coding for a protein having the enzymatic activity of a debranching enzyme or at least part of such a protein and being fused to the 3′ end of the promoter in anti-sense orientation; and

[0071] (iii) optionally a termination signal for the transcription termination and the addition of a poly-A tail to the transcript formed that is coupled to the 3′ end of the DNA sequence defined in (ii); and

[0072] (b) transformation of plant cells with the expression cassette produced in step (a).

[0073] In principle, any promoter that is functional in plants can be used as the promoter mentioned in (i). The promoter may be homologous or heterologous with respect to the plant species used. A suitable promoter is, e.g., the 35S promoter of the cauliflower mosaic virus (Odell et al., Nature 313 (1985), 810-812) which allows constitutive expression in all tissues of a plant, and the promoter construct described in WO94/01571. However, promoters can be used that lead to expression of subsequent sequences only at a certain point of time determined by external factors (see, e.g., WO93/07279) or in a certain tissue of the plant (see, e.g., Stockhaus et al., EMBO J. 8 (1989), 2245-2251). Preferably promoters are used that are active in the starch-storing organs of the plants to be transformed. These starch-storing organs are, e.g., the maize grains in maize while it is the tubers in potato. For the transformation of potato, the tuber-specific B33 promoter (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) is particularly, but not exclusively, useful. Provided that the DNA sequence mentioned under process step (a)(ii) coding for a protein having the enzymatic activity of a debranching enzyme is linked to the promoter in sense orientation, this DNA sequence can be of native or homologous origin or of foreign or heterologous origin with respect to the plant species to be transformed.

[0074] The use of the DNA molecules of the invention is preferred. In principle, the synthesized protein can be localized in any compartment of the plant cell. Debranching enzymes of plants are regularly localized in the plastids and therefore possess a signal sequence for the translocation in these compartments. In the case of the amino acid sequence depicted in Seq ID No. 17 the signal sequence consists of the 64 N-terminal amino acids. In order to achieve localization in another compartment of the cell, the DNA sequence coding for said signal sequence must be removed and the coding region must be linked to DNA sequences allowing localization in the respective compartment. Such sequences are known (see, e.g., Braun et al., EMBO J. 11 (1992), 3219-3227; Wolter et al., Proc. Natl. Acad. Sci. USA 85 (1988), 846-850; Sonnewald et al., Plant J. 1 (1991), 95-106).

[0075] Provided that the DNA sequence mentioned under process step (a) (ii) coding for a protein having the enzymatic activity of a debranching enzyme is linked to the promoter in anti-sense orientation, this DNA sequence preferably is of homologous origin with respect to the plant species to be transformed. However, DNA sequences can also be used that exhibit a high degree of homology to endogenous present debranching enzyme genes, particularly homologies over 80%, preferably homologies between 90% and 100% and most preferably homologies over 95%.

[0076] Sequences down to a minimum length of 15 bp can be used. An inhibiting effect, however, cannot be excluded even if shorter sequences are used. Preferred are longer sequences between 100 and 500 base pairs; for an efficient anti-sense inhibition sequences with a length of more than 500 base pairs are particularly used. Usually, sequences are used that are shorter than 5000 base pairs, preferably sequences that are shorter than 2500 base pairs.

[0077] In the case of the transformation of potato the DNA sequence preferably is the DNA sequence depicted in Seq ID No. 23 or parts thereof that are long enough to produce an anti-sense effect.

[0078] Termination signals for the transcription in plant cells have been described and can be freely interchanged. For example, the termination sequence of the octopin synthase gene from Agrobacterium tumefaciens can be used.

[0079] The transfer of the expression cassette constructed according to process step (a) to plant cells is preferably brought about by using plasmids, particularly plasmids that allow stable integration of the expression cassette into the plant genome.

[0080] In principle, the processes described above can be applied to all plant species. Of interest are both monocotyledonous and dicotyledonous plants. For various monocotyledonous and dicotyledonous plant species transformation techniques have already been described. The processes are preferably applied to useful plants, in particular starch-producing plants, such as cereals (such as maize, wheat, barley, rye, oat), potato, pea, rice, cassava, etc.

[0081] For the preparation of the introduction of foreign genes into taxonomically higher plants there is a wide choice of cloning vectors that contain a replication signal for E. coli and a marker gene for the selection of transformed bacterial cells. Examples of such vectors are pBR322, pUC series, M13mp series, pACYC184, etc. The desired sequence can be introduced into the vector at a suitable restriction site. The plasmid obtained is used to transform E. coli cells. Transformed E. coli cells are cultivated in a suitable medium, harvested and lysed. The plasmid is recovered according to standard techniques. As methods for the analysis for the characterization of the obtained plasmid DNA restriction analyses and sequence analyses are generally used. After each manipulation the plasmid DNA can be cleaved and the resulting DNA fragments can be linked to other DNA sequences.

[0082] There is a large number of techniques available for the introduction of DNA into a plant host cell. These techniques include the transformation of plant cells with T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes as transformant, the fusion of protoplasts, injection, electroporation of DNA, the introduction of DNA via the biolistic technique and other possible techniques.

[0083] In the case of injection and electroporation of DNA into plant cells no specific requirements are made to the plasmids used. Simple plasmids such as pUC derivatives can be used. If, however, one intends to regenerate whole plants from the respectively transformed cells, it is necessary that a selectable marker gene is present.

[0084] Depending on the method of introduction of the desired genes into the plant cell further DNA sequences can be necessary. If, e.g., the Ti or Ri plasmid is used to transform the plant cell, at least the right border, often, however, the right and left border of the Ti and Ri plasmid T-DNA must be linked as flanking region to the genes to be introduced.

[0085] If Agrobacteria are used for transformation, the DNA to be introduced must be cloned into special plasmids, either into an intermediate vector or into a binary vector. The intermediate vectors can be integrated by homologous recombination into the Ti or Ri plasmid of the Agrobacteria due to sequences that are homologous to sequences in the T-DNA. Said plasmid contains the vir region necessary for the transfer of the T-DNA. Intermediate vectors are not able to replicate in Agrobacteria. The intermediate vector can be transferred to Agrobacterium tumefaciens using a helper plasmid (conjugation). Binary vectors are able to replicate both in E. coil and in Agrobacteria. They contain a selection marker gene and a linker or polylinker flanked by the right and left T-DNA border regions. They can be directly transformed into Agrobacteria (Holsters et al., Mol. Gen. Genet. 163 (1978), 181-187). The plasmids used for transformation of Agrobacteria contain furthermore a selection marker gene allowing selection of transformed bacteria, such as the NPTII gene. The Agrobacterium serving as host cell should contain a plasmid carrying a vir region.

[0086] The vir region is necessary for the transfer of the T-DNA to the plant cell. Additional T-DNA may be present. The Agrobacterium so transformed is used to transform plant cells.

[0087] The use of T-DNA for the transformation of plant cells has been extensively examined and is sufficiently described in EP 120516; Hoekema, In: The Binary Plant Vector System, Offsetdrukkerij Kanters B. V., Alblasserdam, Chapter V; Fraley et al., Crit. Rev. Plant. Sci., 4:1-46 and An et al., EMBO J. 4 (1985), 277-287. Some binary vectors are already commercially available, e.g., pBIN19 (Clontech Laboratories, Inc. USA).

[0088] For the transfer of the DNA to the plant cells plant explants can expediently be cocultivated with Agrobacterium tumefaciens or Agrobacterium rhizogenes. From the infected plant material (e.g., pieces of leaves, stem segments, roots but also protoplasts or suspension-cultivated plant cells) whole plants can be regenerated on an appropriate medium which may contain antibiotics or biocides for the selection of transformed cells. The plants thus obtained can be screened for the presence of the introduced DNA.

[0089] Once the introduced DNA is integrated into the genome of the plant cell, it generally remains there stably and can also be found in the successors of the originally transformed cell. Normally it contains a selection marker which imparts to the transformed plant cells resistance to a biocide or an antibiotic such as kanamycin, G 418, bleomycin, hygromycin or phosphinotricin etc. The selected marker should therefore allow for the selection of transformed cells over cells lacking the introduced DNA.

[0090] The transformed cells grow within the cell as usual (cf., e.g., McCormick et al., Plant Cell Reports 5 (1986), 81-84). These plants can be grown in the usual manner and can be cross-bred with plants possessing the same transformed genetic material or other genetic materials. The resulting hybrid individuals have the corresponding phenotypic properties.

[0091] Two or more generations should be cultivated in order to make sure that the phenotypic features are stably retained and inherited. Furthermore, seeds should be harvested in order to make sure that the corresponding phenotype or other characteristics have been retained.

[0092] The introduction of an expression cassette constructed according to the processes described above results in the formation of RNA in the transformed plant cells. If the DNA sequence coding for a debranching enzyme is linked with the promoter in sense orientation in the expression cassette, mRNA is synthesized which may serve as template for the synthesis of an additional or new debranching enzyme in the plant cells. As a result, these cells exhibit an increased debranching enzyme activity leading to a change of the branching degree of the amylopectin produced in the cells. By this change a starch becomes available that excels vis-á-vis the naturally occurring starch by a more coordinate spatial structure and a higher uniformity. This has inter alia favorable effects on the film-forming properties of the starch.

[0093] If the DNA sequence coding for a debranching enzyme is linked with the promoter in anti-sense orientation, an anti-sense RNA is synthesized in transgenic plant cells that inhibits the expression of endogenous debranching enzyme genes. As a result, these cells exhibit a reduced debranching enzyme activity leading to the formation of a modified starch. By using this anti-sense technique it is possible to produce plants in which the expression of endogenous debranching enzyme genes is inhibited in various degrees in a range of 0% to 100%, thereby allowing the production of plants synthesizing amylopectin starch in modified branching degrees. This represents an advantage vis-á-vis conventional breeding and mutagenesis techniques which involve a considerable amount of time and money to provide such a variety. Highly branched amylopectin has a particularly large surface and is therefore specifically suitable as copolymer. A high branching degree furthermore results in an improved solubility in water of the amylopectin. This property is very advantageous for certain technical applications. Particularly suitable for the production of modified amylopectin using the DNA molecules of the invention which code for debranching enzymes is the potato. However, the use of the invention is not limited to this plant species.

[0094] The modified starch synthesized in the transgenic plants can be isolated from the plants or plant cells according to conventional methods and can be used for the production of food and industrial products once it is purified.

[0095] Due to its properties, the starch obtainable from the plant cells and/or plants of this invention is suitable for various industrial applications.

[0096] Basically, starch can be subdivided into two major categories, namely hydrolysis products of starch and what are called native starches. Hydrolysis products essentially include glucose obtained by enzymatic or chemical processes as well as glucose building blocks which can be used for further processes, such as fermentation, or further chemical modifications. In this context, it might be of importance that the hydrolysis process can be carried out simply and inexpensively, currently, it is craried out substantially enzymatically using amyloglucosidase. It is thinkable that costs may be reduced by using lower amounts of enzymes for hydrolysis due to changes in the starch structure, e.g. increase of surface of the grain, improved digestability due to less branching or steric structure, which limits the accessibility for the used enzymes.

[0097] The use of what are called native starches, which are used because of their polymer structure, can be subdivided into two large areas:

[0098] (a) Use in Foodstuffs

[0099] Starch is a classic additive for various foodstuffs, in which it essentially serves the purpose of binding aqueous additives and/or causes an increased viscosity or an increased gel formation. Important characteristic properties are flowing and sorption behavior, swelling and pastification temperature, viscosity and thickening performance, solubility of the starch, transparency and paste structure, heat, shear and acid resistance, tendency to retrogradation, capability of film formation, resistance to freezing/thawing, digestibility as well as the capability of complex formation with e.g. inorganic or organic ions.

[0100] (b) Use in Non-Foodstuffs

[0101] The other major field of application is in the use of starch as an adjuvant in various production processes and/or as an additive in technical products. The major fields of application for the use of starch as an adjuvant are, first of all, the paper and cardboard industries. In this field, the starch is mainly used for retention (holding back solids), for sizing filler and fine particles, as solidifying substance and for dehydration. In addition, the advantageous properties of starch with regard to stiffness, hardness, sound, grip, gloss, smoothness, tear strength as well as the surfaces are utilized.

[0102] Within the paper production process, a differentiation can be made between four fields of application, namely surface, coating, mass and spraying.

[0103] The requirements on starch with regard to surface treatment are essentially a high degree of brightness, corresponding viscosity, high viscosity stability, good film formation as well as low formation of dust. When used in coating, the solid content, a corresponding viscosity, a high capability to bind as well as a high pigment affinity are important. As an additive to the mass, rapid, uniform, loss-free dispersion, high mechanical stability and complete retention in the paper pulp are of importance. When using the starch in spraying, corresponding content of solids, high viscosity as well as high capability to bind are also of importance.

[0104] A major field of application is, for instance, in the adhesive industry, where the fields of application are subdivided into four areas: the use as pure starch glue, the use in starch glues prepared with special chemicals, the use of starch as an additive to synthetic resins and polymer dispersions as well as the use of starches as extenders for synthetic adhesives. 90% of all starch-based adhesives are used in the production of corrugated board, paper sacks, bags, composite materials for paper and aluminum, boxes and wetting glue for envelopes, stamps, etc.

[0105] Another possible use as adjuvant and additive is in the production of textiles and textile care products. Within the textile industry, a differentiation can be made between the following four fields of application: the use of starch as a sizing agent, i.e. as an adjuvant for smoothing and strengthening the burring behavior for the protection against tensile forces active in weaving as well as for the increase of wear resistance during weaving, as an agent for textile improvement mainly after quality-deteriorating pretreatments, such as bleaching, dying, etc., as thickener in the production of dye pastes for the prevention of dye diffusion and as an additive for warping agents for sewing yarns.

[0106] Furthermore, the starch may be used as an additive in building materials. One example is the production of gypsum plaster boards, in the course of which the starch mixed in the thin plaster pastifies with the water, diffuses at the surface of the gypsum board and thus binds the cardboard to the board. Other fields of application are admixing it to plaster and mineral fibers. In ready-mixed concrete, starch may be used for the deceleration of the sizing process.

[0107] Furthermore, the starch is advantageous for the production of means for ground stabilisation used for the temporary protection of ground particles against water in artificial earth shifting. According to state-of-the-art knowledge, combination products consisting of starch and polymer emulsions can be considered to have the same erosion- and incrustation-reducing effect as the products used so far; however, they are considerably less expensive.

[0108] Furthermore, the starch may be used in plant protectives for the modification of the specific properties of these preparations. For instance, starches are used for improving the wetting of plant protectives and fertilizers, for the dosed release of the active ingredients, for the conversion of liquid, volatile and/or odorous active ingredients into microcristalline, stable, deformable substances, for mixing incompatible compositions and for the prolongation of the duration of the effect due to a reduced disintegration.

[0109] Another important field of application lies in the fields of drugs, medicine and the cosmetics industry. In the pharmaceutical industry, the starch may be used as a binder for tablets or for the dilution of the binder in capsules. Furthermore, starch is suitable as disintegrant for tablets since, upon swallowing, it absorbs fluid and after a short time it swells so much that the active ingredient is released. Medicinal flowance and dusting powders are further fields of application. In the field of cosmetics, the starch may for example be used as a carrier of powder additives, such as scents and salicylic acid. A relatively extensive field of application for the starch is toothpaste.

[0110] Also the use of starch as an additive to coal and briquettes is thinkable. By adding starch, coal can be quantitatively agglomerated and/or briquetted in high quality, thus preventing premature disintegration of the briquettes. Barbecue coal contains between 4 and 6% added starch, calorated coal between 0,1 and 0,5%.

[0111] Furthermore, the starch is suitable as a binding agent since adding it to coal and briquette can considerably reduce the emission of toxic substances.

[0112] Furthermore, the starch may be used as a flocculent in the processing of ore and coal slurry.

[0113] Another field of application is the use as an additive to process materials in casting. For various casting processes, cores produced from sands mixed with binding agents are needed. Nowadays, the most commonly used binding agent is bentonite with modified starches, mostly swelling starches, mixed in.

[0114] The purpose of adding starch is increased flow resistance as well as improved binding strength.

[0115] Moreover, swelling starches may fulfil more prerequisites for the production process, such as dispersability in cold water, rehydratisability, good mixability in sand and high capability of binding water.

[0116] In the rubber industry, the starch may be used for improving the technical and optical quality. Reasons for this are improved surface gloss, grip and appearance.

[0117] For this purpose, the starch is dispersed on the sticky rubberized surfaces of rubber substances before the cold vulcanisation. It may also be used for improving the printability of rubber.

[0118] Another field of application for the modified starch is the production of leather substitutes.

[0119] In the plastics market, the following fields of application are emerging: the integration of products derived from starch into the processing process (starch is only a filler, there is no direct bond between synthetic polymer and starch) or, alternatively, the integration of products derived from starch into the production of polymers (starch and polymer form a stable bond).

[0120] The use of the starch as a pure filler cannot compete with other substances such as talcum. This situation is different when the specific starch properties become effective and the property profile of the end products is thus clearly changed. One example is the use of starch products in the processing of thermoplastic materials, such as polyethylene. In this process, starch and synthetic polymer are combined in a ratio of 1:1 by means of coexpression to form a ‘master batch’, from which various products are produced by means of common techniques using granulated polyethylene. The integration of starch in polyethylene films may cause an increased substance permeability in hollow bodies, improved water vapor permeability, improved antistatic behavior, improved anti-block behavior as well as improved printability with aqueous dyes.

[0121] Another possibility is the use of the starch in polyurethane foams. Due to the adaptation of starch derivatives as well as due to the optimisation of processing techniques, it is possible to control the reaction between synthetic polymers and the starch's hydroxy groups. The result are polyurethane films having the following property profiles due to the use of starch: a reduced coefficient of thermal expansion, decreased shrinking behavior, improved pressure/tension behavior, increased water vapor permeability without a change in water acceptance, reduced flammability and cracking density, no drop off of combustible parts, no halides and reduced aging. Disadvantages that presently still exist are reduced resistance to pressure and to impact.

[0122] Product development of film is not the only option. Also solid plastics products, such as pots, plates and bowls can be produced having a starch content of more than 50%. Furthermore, the starch/polymer mixtures offer the advantage that they are much easier biodegradable.

[0123] Furthermore, due to their extreme capability to bind water, starch graft polymers have gained utmost importance. These are products having a backbone of starch and a side lattice of a synthetic monomer grafted on according to the principle of radical chain mechanism. The starch graft polymers available today are characterized by an improved binding and retaining capability of up to 1000 g water per g starch at a high viscosity. These super absorbers are used mainly in the hygiene field, e.g. in products such as diapers and sheets, as well as in the agricultural sector, e.g. in seed pellets.

[0124] What is decisive for the use of the new starch modified by genetic engineering are, on the one hand, structure, water content, protein content, lipid content, fiber content, ashes/phosphate content, amylose/amylopectin ratio, distribution of the relative molar mass, degree of branching, granule size and form as well as crystallisation, and on the other hand, the properties resulting in the following features: flow and sorption behavior, pastification temperature, thickening performance, solubility, paste structure, transparency, heat, shear and acid resistance, tendency to retrogradation, capability of gel formation, resistance to freezing/thawing, capability of complex formation, iodine binding, film formation, adhesive strength, enzyme stability, digestibility and reactivity. Furthermore, viscosity is particularly pointed out.

[0125] Furthermore, the modified starch obtainable from the plant cells and/or plants of this invention may be subjected to further chemical modification, which will result in further improvement of the quality for certain of the above-described fields of application or in a new field of application. These chemical modifications are principally known to the person skilled in the art. These are particularly modifications by means of

[0126] acid treatment

[0127] oxidation

[0128] esterification (formation of phosphate, nitrate, sulfate, xanthate, acetate and citrate starches; other organic acids may also be used for the esterification)

[0129] formation of starch ethers (starch alkyl ether, O-allyl ether, hydroxylalkyl ether, O-carboxylmethyl ether, N-containing starch ethers, S-containing starch ethers)

[0130] formation of branched starches

[0131] formation of starch graft polymers.

[0132] The invention furthermore relates to the use of the DNA molecules of the invention for the production of plants synthesizing an amylopectin starch having a modified branching degree as compared to that of wild type plants.

[0133] A further subject matter of the present invention is the use of the DNA molecules of the invention or of parts of these DNA molecules or of the reverse complements of these molecules for the identification and isolation of homologous molecules coding for proteins having the enzymatic activity of a debranching enzyme, or of fragments of such proteins, from plants or other organisms. For a definition of the term “homology” see above.

[0134] FIG. 1 shows plasmid pDBE-Spi

[0135] The thin line corresponds to the sequence of pBluescriptSKII(-). The thick line represents the cDNA coding for the debranching enzyme of Spinacia oleracea. The cDNA insert is ligated between the EcoRI and XhoI restriction sites of the polylinker of the plasmid. The arrow indicates the orientation of the coding region for the debranching enzyme. The DNA sequence of the cDNA insert is indicated in Seq ID No. 17.

[0136] FIG. 2 shows plasmid p35S-DBE-Spi

[0137] A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al, Cell 21 (1980), 285-294)

[0138] B=fragment B: cDNA from spinacia oleracea coding for the debranching enzyme;

[0139] EcoRI/XhoI fragment from pDBE-Spi, about 3440 bp; orientation towards the promoter: sense

[0140] C=fragment C: nt 11748-11939 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846)

[0141] FIG. 3 shows the purification of the debranching enzyme from Solanum tuberosum.

[0142] 0=protein extract of a homogenate from tuber tissue of Solanum tuberosum

[0143] Durchlauf=void volume of an affinity chromatography of the protein extract on immobilized β-cyclodextrin

[0144] β-cyclodextrin=elution of the affinity chromatography with dissolved β-cyclodextrin in a concentration of 1 mg/ml or 10 mg/ml

[0145] DBE=debranching enzyme from Solanum tuberosum

[0146] DE=disproportionating enzyme from Solanum tuberosum

[0147] FIG. 4 shows plasmid pDBE-Pot

[0148] The thin line corresponds to the sequence of pBluescriptSKII(-). The thick line represents the partial cDNA sequence coding for part of the debranching enzyme from Solanum tuberosum. The arrow indicates the orientation of the coding region for the debranching enzyme. The cDNA insert is ligated between the BamHI and SmaI restriction sites of the polylinker of the plasmid. The DNA sequence of the cDNA insert is indicated in Seq ID No. 23.

[0149] FIG. 5 shows plasmid p35S-antiDBE-Pot

[0150] A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al, Cell 21 (1980), 285-294)

[0151] B=fragment B: partial cDNA sequence from Solanum tuberosum coding for a part of the debranching enzyme;

[0152] SmaI/BamHI fragment from pDBE-Pot, about 500 bp; orientation towards the promoter: anti-sense

[0153] C=fragment C: nt 11748-11939 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846)

[0154] FIG. 6 shows an activity gel regarding the debranching enzyme from tuber extracts of potato plants (RE7 series) transformed with plasmid p35S-DBE-Spi and wild type plants.

[0155] DBE=debranching enzyme

[0156] W=wild type plant

[0157] The numbers indicate the plant lines examined.

[0158] FIG. 7 shows curves of aqueous solutions of starch isolated from potato plants that were recorded with a Rapid Visco Analyser. Curves 1 to 3 indicate the viscosities of the solutions of starch obtained from the potato plants transformed with plasmid p35S-DBE-Spi. For a comparison, curve 4 represents the viscosity profile of the wild type starch.

[0159] Curve 1: plant line RE7-34,

[0160] Curve 2: plant line RE7-26,

[0161] Curve 3: plant line RE7-10,

[0162] Curve 4: wild type starch.

[0163] FIG. 8 shows the stability of the starch gels produced. The curves illustrate the force that is necessary to deform the gel. The force profiles 1 to 3 correspond to the starch that was obtained from the potato plants transformed with plasmid p35S-DBE-Spi. For a comparison, curve 4 indicates the force profile of the wild type starch;

[0164] Curve 1: plant line RE7-34,

[0165] Curve 2: plant line RE7-26,

[0166] Curve 3: plant line RE7-10,

[0167] Curve 4: wild type starch.

[0168] FIG. 9 shows an activity gel regarding the debranching enzyme from tuber extracts of potato plants (RE500 series) transformed with plasmid p35S-antiDBE-Pot and wild type plants.

[0169] DBE=debranching enzyme

[0170] W=wild type

[0171] The numbers indicate the plant lines examined.

[0172] FIG. 10 Microscopic photograph of a starch granule from a potato plant transformed with plasmid p35S-antiDBE-Pot.

[0173] FIG. 11 shows curves of aqueous solutions of starch isolated from potato plants that were recorded with a Rapid Visco Analyser. Curves 1 to 3 indicate the viscosities of the solutions of starch obtained from the potato plants transformed with plasmid p35S-antiDBE-Pot. For a comparison, curve 4 represents the viscosity profile of the wild type starch.

[0174] Curve 1: plant line RE500-47,

[0175] Curve 2: plant line RE500-75,

[0176] Curve 3: plant line RE500-81,

[0177] Curve 4: wild type starch.

[0178] FIG. 12 shows the stability of the starch gels produced. The curves illustrate the force that is necessary to deform the gel. The force profiles 1 to 3 correspond to the starch that was obtained from the potato plants transformed with plasmid p35S-antiDBE-Pot. For a comparison, curve 4 indicates the force profile of the wild type starch:

[0179] Curve 1: plant line RE500-47,

[0180] Curve 2: plant line RE500-75,

[0181] Curve 3: plant line RE500-81,

[0182] Curve 4: wild type starch.

[0183] The examples serve to illustrate the invention.

[0184] In the following examples, the following techniques are used:

[0185] 1. Cloning Techniques

[0186] For the cloning in E. coli the vectors pBluescriptSKII(-) (Stratagene) and pUC19 were used.

[0187] For the plant transformation the gene constructs were cloned into the binary vector pBinAR.

[0188] 2. Bacterial Strains

[0189] For the Bluescript vectors, the pUC vectors and for the pBinAR constructs the E. coli strain DH5α (Bethesda Research Laboratories, Gaithersburgh, USA) were used. For in-vivo excision the E. coli strain XL1-Blue was used.

[0190] Transformation of the plasmids in the potato plants was carried out by using Agrobacterium tumefaciens strain C58C1 pGV2260 (Deblaere et al., Nucl. Acids Res. 13 (1985), 4777-4788).

[0191] 3. Transformation of Agrobacterium tumefaciens

[0192] Transfer of the DNA was carried out by direct transformation according to the method by Höfgen and Willmitzer (Nucleic Acids Res. 16 (1988), 9877). The plasmid DNA of transformed Agrobacteria was isolated according to the method by Birnboim and Doly (Nucleic Acids Res. 7 (1979), 1513-1523) and subjected to gel electrophoretic analysis after suitable restriction.

[0193] 4. Transformation of Potatoes

[0194] Ten small leaves of a potato sterile culture (Solanum tuberosum L.cv. Désirée) were wounded with a scalpel and placed in 10 ml MS medium (Murashige and Skook, Physiol. Plant. 15 (1962), 473) containing 2% sucrose which contained 50 μl of a selectively grown overnight culture of Agrobacterium tumefaciens. After gently shaking the mixture for 3-5 minutes it was further incubated in the dark for 2 days. For callus induction the leaves were placed on MS medium containing 1.6% glucose, 5 mg/l naphthyl acetic acid, 0.2 mg/l benzyl aminopurine, 250 mg/l claforan, 50 mg/l kanamycin, and 0.80% Bacto agar. After incubation at 25° C. and 3,000 lux for one week the leaves were placed for shoot induction on MS medium containing 1.6% glucose, 1.4 mg/l zeatin ribose, 20 mg/l naphthyl acetic acid, 20 mg/l giberellic acid, 250 mg/l claforan, 50 mg/l kanamycin and 0.80% Bacto agar.

[0195] 5. Radioactive Labelling of DNA Fragments

[0196] The DNA fragments were radioactively labelled using a DNA Random Primer Labelling Kit of Boehringer (Germany) according to the manufacturer's information.

[0197] 6. Northern Blot Analysis

[0198] RNA was isolated according to standard techniques from leave tissue of plants. 50 μg of RNA were separated in an agarose gel (1.5% agarose, 1×MEN buffer, 16.6% formaldehyde). The gel was shortly rinsed with water after gel run. The RNA was transferred with 20×SSC by capillary blot on a Hybond N nylon membrane (Amersham, UK). The RNA was then fixed on the membrane by UV cross-linking.

[0199] The membrane was prehybridized in NSEB buffer at 68° C. for 2 hrs and was then hybridized in NSEB buffer at 68° C. overnight in the presence of the radioactively labelled probe.

[0200] 7. Plant Cultivation

[0201] Potato plants were cultivated in a greenhouse under the following conditions:

	Light period	16	hrs at 25,000 lux and 22° C.
	Dark period	8	hrs at 15° C.
	Humidity	60%

[0202] 8. Detection of Debranching Enzymes in a Native Gel

[0203] For detection of debranching enzyme activity by non-denaturing gel electrophoresis tissue samples of potato tubers were broken up in 50 mM Tris-HCl (pH 7.6), 2 mM DTT, 2.5 mM EDTA, 10% glycerol and 0.4 mM PMSF. Electrophoresis was carried out in a MiniProtean II chamber (BioRad) under non-denaturing conditions according to Laemmli (Nature 227 (1970), 680-685). The monomer concentration of the gels with 1.5 mm thickness was 7.5% (w/v), and the gel contained 1% red pullulan (Megazyme, Australia). Equivalent amounts of protein extract were applied and separated for 2 hrs at 10 mA per gel.

[0204] The activity gels were then incubated in 100 mM sodium acetate (pH 6.0) and 0.1% β-mercaptoethanol at 37° C.

[0205] Debranching enzyme activity was detected by hydrolysis of the red pullulan (decoloring).

[0206] 9. Starch Analysis

[0207] The starch produced by the transgenic potato plants was characterized by the following methods:

[0208] a) Determination of the Phosphate Content

[0209] In potato starch some glucose units may be phosphorylated at the carbon atoms at positions C3 and C6. In order to determine the phosphorylation degree at the C6 position of the glucose 100 mg starch were hydrolyzed in 1 ml 0.7 M HCl at 95° C. for 4 hours (Nielsen et al., Plant Physiol. 105 (1994), 111-117).

[0210] After neutralization with 0.7 M KOH, 50 μl of the hydrolysate were subjected to a photometric-enzymatic test to determine the glucose-6-phosphate content. The change of the absorption of the test mixture (100 mM imidazol/HCl; 10 mM MgCl2; 0.4 mM NAD; 2 units glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides; 30° C.) was measured at 334 nm.

[0211] b) Determination of the Amylose/Amylopectin Ratio in Starch from Potato Plants

[0212] Starch was isolated from potato plants according to standard techniques and the amylose/amylopectin ratio was determined according to the method described by Hovenkamp-Hermelink et al. (Potato Res. 31 (1988), 241-246).

[0213] c) Determination of the Viscosity of the Aqueous Solution of the Starch

[0214] In order to determine the viscosity of the aqueous solutions of the starch synthesized by the transformed potato plants starch was isolated from tubers of transformed plants. 2 g starch each were dissolved in 25 ml H2O and used for analysis in a Rapid Visco Analyser (Newport Scientific Pty Ltd., Investment Support Group, Warriewood NSW 2102, Australia). The analyser was operated as indicated by the manufacturer. In order to determine the viscosity of the aqueous solution of the starch the starch suspension was first heated from 50° C. to 96° C. at a speed of 12° C. per minute. Subsequently, the temperature was kept at 96° C. for 2.5 min. Afterwards, the solution was cooled from 96° C. to 50° C. at a speed of 16.4° C. per minute. During the entire test the viscosity was measured. Representative results of these measurements are illustrated as curves showing the viscosity in dependence of time.

[0215] d) Determination of the Stability of the Gel

[0216] For a further characterization of the starch its gel stability was measured using a TA-XT2 Texture Analyser (Stable Micro Systems, Unit 105, Blackdown Rural Industries, Haste Hill, Haslemere, Surrey Gu 27 3AY, England) following the manufacturer's recommendations.

[0217] The paste obtained from the Rapid Visco Analyser (see 9c) was stored at room temperature for 24 hours to form a gel. Then the gel stability was measured by allowing a plastic probe (d=10 mm) to penetrate the gel sample of 2 cm thickness at a speed of 0.5 mm/sec to a depth of up to 7 mm. The measurement yielded characteristic force profiles in dependence of time.

[0218] Abbreviations Used

EDTA ethylene diamine tetraacetic acid
IPTG isopropyl β-D-thiogalacto-pyranoside
PAAG polyacrylamide gel
PCR  Polymerase Chain Reaction
PMSF phenylmethylsulfonyl fluoride
SDS  sodium dodecyl sulphate

[0219] Materials and Solutions Used

	Buffer A	50	mM	sodium acetate (pH 6.0)
		2.5	mM	1,4-dithio-DL-threitol
		1.5	mM	β-mercaptoethanol
		0.4	mM	PMSF
		traces of sodium bisulfite, sodium sulfite and
		ascorbic acid
	Buffer B	50	mM	MES
		pH	5.8	with NaOH
	Buffer C	250	mM	phosphate buffer
		250	mM	NaCl
		1	mM	EDTA
		7%		SDS
		25%		PEG6000
		25%		formamide
		250	mg/l	denatured herring sperm DNA
	Buffer H	2 ×		SSC
		10 ×		Denhardt's solution
		0.1%		SDS
		5	mM	EDTA
		50	mM	disodium phosphate
		250	mg/l	herring sperm DNA
		50	mg/ml	tRNA
	20 × SSC	175.3	g	NaCl
		88.2	g	sodium citrate
		ad	1000 ml with ddH2O
		pH	7.0 with 10 N NaOH

EXAMPLE 1

[0220] Purification of a Debranching Enzyme from Spinacia oleracea and Determination of Peptide Sequences

[0221] For purification of a debranching enzyme from spinach 500 g “deribbed” spinach leaves were homogenized in 1.5 l buffer B in a “Warring blender” for 1 min. The homogenate was strained through a sandwich of six layers of muslin cloth.

[0222] If necessary, the pH was adjusted to a value of 5.8. The homogenate was subjected to centrifugation at 25,000×g for 30 min. Then proteins were precipitated from the supernatant by ammonium sulphate precipitation at 0° C. For this purpose, ammonium sulphate was continually added under stirring to the supernatant up to a concentration of 40% of the saturation concentration. Precipitation of proteins was continued under stirring for another 30 min. After separation of the precipitated proteins by centrifugation at 25,000×g for 30 min the resulting supernatant was mixed with ammonium sulphate up to a concentration of 50% of the saturation concentration. Precipitation was carried on for another 30 min. Then the mixture was centrifuged at 25,000×g for 30 min and the precipitated proteins were resuspended in about 80 ml buffer B. After centrifugation at 30,000×g for 15 min the supernatant was removed and applied to a affinity column which had been equilibrated with buffer B.

[0223] The material of the affinity column was expoxy-activated sepharose 6B (Sigma) to which β-cyclodextrin (Cycloheptaamylose; Sigma) had been coupled as described by Ludwig et al. (Plant Physiol. 74 (1984), 856-861). The affinity column was washed with buffer B until the eluate did not exhibit any absorption at 280 nm. Elution of the debranching enzyme was carried out with 1 mg/ml β-cyclodextrin in buffer B. The fractions were assayed for debranching enzyme activity with the DNSS test (see Ludwig et al., loc. cit.) and were pooled. Removal of the cyclodextrin and concentration of the protein solution was carried out with Centricon 30 tubes (Amicon), with several washings with buffer B.

[0224] 200 μg of the purified protein were cleaved with BrCN as described in Matsudaira (“A Practical Guide to Protein and Peptide Purification for Microsequencing”, Academic Press, Inc., San Diego (1989), page 29). The resulting peptides were separated via SDS polyacrylamide gel electrophoresis.

[0225] The gel used was a gradient gel in which the polyacrylamide concentration increased from 12% to 18% at a constant urea concentration of 6 M. The peptides were transferred from the SDS gel to a PVDF membrane by semi-dry-electroblotting.

[0226] Peptide sequences of the isolated peptides were determined according to standard techniques. Two of the identified peptide sequences of the debranching enzyme of Spinacia oleracea are depicted in Seq ID No. 13 and Seq ID No. 14.

EXAMPLE 2

[0227] Isolation of a cDNA Coding for a Debranching Enzyme from Spinach

[0228] The peptide sequences of the debranching enzyme from spinach as obtained according to Example 1 were used to derive oligonucleotide sequences that represent regions of the DNA sequence of the gene coding for a debranching enzyme from spinach when considering the degeneracy of the genetic code.

[0229] In accordance with the derived oligonucleotide sequences synthetic oligonucleotides were synthesized according to standard techniques. These oligonucleotides were used for amplification by PCR using mRNA from spinach leaves as template.

[0230] In order to allow for as efficient as possible hybridization of the oligonucleotides to the desired DNA fragment oligonucleotides with as great as possible length should be used. However, with increasing length the degree of degeneracy also increases, i.e., the number of oligonucleotides having different sequence combinations. Degrees of degeneracy of up to 6,000 are acceptable.

[0231] For the peptide sequence depicted in Seq ID No. 13 the sequence was derived for an oligonucleotide probe having a length of 20 bp. The oligonucleotide has a GC content of maximal 40% and minimal 30%. For the peptide sequence depicted in Seq ID No. 14 the sequence was derived for an oligonucleotide probe having a length of 20 bp. The oligonucleotide has a GC content of maximal 45% and minimal 35%.

[0232] The Sequence Template for the Synthesis of Oligonucleotides are:

Peptide A:	NH2-Gln Pro Ile Glu Thr Ile Asn Tyr Val-COOH	(Seq ID No. 13)
RNA:	5′ CAR CCN AUH GAR ACN AUH AAY UAY GUN 3′
Oligo A:	3′ TAC GTY GGW TAR CTY TGW TA 5′	(Seq ID No. 15)
Peptide B:	NH2-Asn Ile Asp Gly Val Glu Gly-COOH	(Seq ID No. 14))
mRNA:	5′ AAY AUH GAY GGN GUN GAR GGN 3′
Oligo B:	5′ AAY ATY GAT GGW GTI GAR GG 3′	(Seq ID No. 16)

[0233] The RNA was isolated from spinach leaves according to standard techniques. It was used for PCR using oligonucleotides A and B. 100 pmol each of the oligonucleotides per reaction mixture were used at an annealing temperature of 52° C. and a MgCl2 concentration of 1.5 mM. 30 cycles were carried out. PCR yielded a DNA fragment of about 500 bp.

[0234] This DNA fragment was ligated into vector pUC19 which was cleaved with SmaI. The resulting plasmid was called pAR1. A part of the sequence of the DNA insert was determined using the method of Sanger et al. (Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The determined sequence corresponds to nucleotides 1920 to 2216 of the DNA sequence depicted in Seq ID No. 17 except for the mismatches introduced by the degenerate oligonucleotides. The amino acid sequence derived from said sequence exhibits an identity of 42.2% to the pulA gene from Klebsiella aerogenes coding for a pullulanase over a range of 90 amino acids.

[0235] For isolation of cDNA molecules coding for the debranching enzyme of spinach a cDNA library was constructed in the vector Lambda ZAP II (Stratagene) as described by Sonnewald et al. (Planta 189 (1993), 174-181) and was packaged in phage heads. Subsequently, E. coli cells of the XL1-Blue strain were infected with the phages containing the cDNA fragments and plated on a medium in Petri dishes in a density of about 30,000 per 75 cm2. After about 8 hrs of incubation nitrocellulose membranes were placed on the lysed bacterial layer and were removed after one minute. The filters were incubated for 2 min in 0.2 M NaOH; 1.5 M NaCl, then for 2 min in 0.4 M Tris/HCl (pH 7.5) and finally for 2 min in 2×SSC. The filters were dried, UV cross-linked and incubated at 42° C. for 3 hrs in buffer C before the radioactively labelled probe was added. As probe the cDNA insert of plasmid pAR1 was used. Hybridization was carried out at 42° C. for 12 to 16 hrs. The filters were then washed at 45° C. once for 15 min in 1×SSC/0.3% SDS and three times for 15 min in 0.1×SSC/0.3% SDS and then subjected to autoradiography.

[0236] Positive phage clones were individualized and further purified by standard techniques. The in-vivo excision method was used to obtain from positive phage clones E. coli clones containing a double-stranded pBluescript plasmid having the corresponding cDNA insert. After examination of the size and restriction pattern of the inserts the DNA sequence of suitable clones was determined. Several clones were identified that contain inserts coding for the debranching enzyme from spinach, particularly a clone exhibiting a cDNA insert including nucleotides 1804 to 3067 of the DNA sequence depicted in Seq ID No. 17. However, no complete clones were obtained.

[0237] In order to isolate cDNA molecules comprising the entire coding region for the debranching enzyme, a specific cDNA library with mRNA from spinach leaves was established. For this purpose total RNA from spinach leaves was isolated according to standard techniques. In order to obtain poly(A+) mRNA the total RNA was applied to a poly(dT) column from which poly(A+) mRNA was eluted.

[0238] Starting from the poly(A+) mRNA cDNA was prepared according to the method of Gubler and Hoffmann (Gene 25 (1983), 263-269) using an XhoI oligo d(t)18 primer. The cDNA was cleaved with XhoI after EcoRI linker addition and ligated in an oriented manner into a Lambda Uni-ZAP XR vector (Stratagene) which had been cleaved with EcoRI and XhoI. About 2,000,000 plaques of a cDNA library so constructed were screened for cDNA sequences coding for the debranching enzyme. For this purpose E. coli cells of the XL1-Blue strain were infected with the phages containing the cDNA fragments and plated on a medium in Petri dishes in a density of about 30,000 per 75 cm2. After about 8 hrs of incubation nitrocellulose membranes were placed on the lysed bacterial layer and were removed after one minute. The filters were incubated for 2 min in 0.2 M NaOH; 1.5 M NaCl, then for 2 min in 0.4 M Tris/HCl (pH 7.5) and subsequently for 2 min in 2×SSC. The filters were dried, UV cross-linked and incubated at 42° C. for 3 hrs in buffer C before the radioactively labelled probe was added. As probe either the cDNA inserts described above that contained only parts of the coding region for the debranching enzyme, or the insert of plasmid pAR1 was used. After hybridization at 42° C. for 12 hrs the filters were washed at 45° C. once for 15 min in 1×SSC/0.3% SDS and three times for 15 min in 0.1×SSC/0.3% SDS and then subjected to autoradiography.

[0239] Positive phage clones were individualized and further purified by standard techniques. The in-vivo excision method was used to obtain from positive phage clones E. coli clones containing a double-stranded pBluescript plasmid having the corresponding cDNA insert. After examination of the size and restriction pattern of the inserts the plasmid DNA of suitable clones was isolated and the DNA sequence of the cDNA insert was determined.

EXAMPLE 3

[0240] Sequence Analysis of the cDNA Insert of Plasmid pDBE-Spi

[0241] From an E. coli clone obtained according to Example 2 plasmid pDBE-Spi (FIG. 1) was isolated and its cDNA insert determined according to standard techniques using the didesoxy method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert has a length of 3437 bp. The nucleotide sequence and the derived amino acid sequence is indicated in Seq ID No. 17.

EXAMPLE 4

[0242] Construction of Plasmid p35S-DBE-Spi, Transformation of Potato Plants as well as Characterization of the Synthesized Starch

[0243] From plasmid pDBE-Spi a DNA fragment of about 3450 bp length was obtained by EcoRI/XhoI digestion which exhibits the sequence indicated in Seq ID No. 17 and contains the coding region for the spinach debranching enzyme. This DNA fragment was cloned into the vector pBinAR (Höfgen and Willmitzer, Plant Sci. 66 (1990), 221-230) which had been cleaved with SmaI. The vector pBinAR is a derivative of the binary vector pBin19 (Bevan, Nucl. Acids Res. 12 (1984), 8711-8721).

[0244] Construction of plasmid pBinAR is described in Example 12. The resulting plasmid was called p35S-DBE-Spi and is depicted in FIG. 2.

[0245] Insertion of the cDNA fragment results in an expression cassette which consists of fragments A, B and C as follows (FIG. 2):

[0246] Fragment A (529 bp) contains the 35S promoter of cauliflower mosaic virus (CaMV). The fragment includes nucleotides 6909 to 7437 of CaMV (Franck et al., Cell 21 (1980), 285-294).

[0247] Fragment B comprises the protein-encoding region as well as the flanking regions of the cDNA coding for spinach debranching enzyme. It was isolated as EcoRI/XhoI fragment from pDBE-Spi as described above and fused to the promoter in pBinAR in sense orientation.

[0248] Fragment C (192 bp) contains the polyadenylation signal of the gene 3 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).

[0249] The size of plasmid p35S-DBE-Spi is about 14.2 kb.

[0250] Vector p35S-DBE-Spi was transferred to potato plant cells via Agrobacterium tumefaciens-mediated transformation. Intact plants were regenerated from the transformed cells.

[0251] Analysis of RNA for the presence of an mRNA coding for the spinach debranching enzyme can be used to verify if the genetic modification was successfully established. For this purpose, usually a northern blot analysis is made. Total RNA is isolated from plant tissue as described by Logemann et al. (Anal. Biochem. 163 (1987), 16-20), separated via gel electrophoresis, transferred to a nylon membrane and hybridized to a suitable probe.

[0252] As a result of the transformation transgenic potato plants showed increased debranching enzyme activity (cf. FIG. 6).

[0253] The starch produced by these plants is subsequently analyzed and characterized. The starch produced by the transgenic plants differs from the starch synthesized by wild type plants as regards its viscosity and gel stability.

[0254] The viscosity was determined using a Rapid Visco Analyser according to the method described above. The results are shown in FIG. 7. FIG. 7 shows in curve 4 a typical RVA-curve for starch isolated from wild type plants of the potato variety Désirée. Curves 1 to 3 of the transformed plant lines have a considerably less marked to no viscosity maximum at all after heating to 96° C. as well as a higher increase in viscosity after cooling to 50° C. The maximum viscosity of the starch from the transgenic p35S-DBE-Spi plants has a substantially reduced value as compared to starch from wild type plants. The final viscosity of the modified starch after subsequent cooling is considerably higher than the values for starch synthesized in wild type plants.

[0255] FIG. 8 illustrates the gel stability of gels prepared from starch of transgenic plant lines as compared to gels from wild type starch. The gel stability of the modified starch differs considerably. The force that is necessary to deform gels of modified starch is greater than the force that is necessary to deform a corresponding gel prepared from wild type starch.

[0256] The phosphate content of the starch produced in the transgenic plants approximately corresponds to the value for the starch produced in wild type plants (see Table 1). The measuring inaccuracy is about ±5%.

[0257] The amylose content can be calculated according to Hovenkamp-Hermelink et al. (Potato Res. 31 (1988), 241-246).

[0258] The amylose content is slightly increased by 5 to 40% (see Table 1).

TABLE 1
Plants	nmol glucose-6-phosphate/mg starch	% amylose
Wild type	9.00	20.4
RE7-10	10.80	21.7
RE7-26	3.23	21.8
RE7-34	8.49	24.3

[0259] The plant line RE7-34 shows the most significant deviations from the wild type plant.

EXAMPLE 5

[0260] Identification and Isolation of Genomic DNA Sequences Coding for a Debranching Enzyme of Solanum tuberosum

[0261] Identification and isolation of genomic DNA sequences coding for a debranching enzyme of Solanum tuberosum was carried out such that first proteins having the enzymatic activity of a debranching enzyme were isolated from potatoes, peptide sequences of these proteins were determined and from these peptide sequences degenerate oligonucleotide sequences were derived that were used for screening genomic libraries. This process is described in the following in detail.

[0262] (a) Identification of a New Debranching Enzyme in Solanum tuberosum

[0263] Identification of a new debranching enzyme in Solanum tuberosum was carried out according to known methods as described below:

[0264] Protein extracts were obtained from tuber tissue of plants of the species Solanum tuberosum. For this purpose, 820 g tuber tissue were homogenized in 1500 ml buffer A. Of this homogenate 50 ml were separated in a PAAG (see lane 0 in FIG. 3). The gel contained 7.5% acrylamide (pH 7.9) which was linked with methylene bisacrylamide up to a degree of 1:75, as well as 1% amylopectin. The buffer system for electrophoresis contained tris/glycin (pH 7.9). After gel run the gel was equilibrated in 50 mM tris/citrate (pH 7.0); 2 mM ascorbic acid at 22° C. for 4 hrs. The gel was stained with Lugol's solution for 15 min. The result of the staining is shown in FIG. 3, lane 0. In addition to the reddish band that is the result of the activity of a branch-introducing enzyme (branching enzyme of disproportionating enzyme) a band with a strong blue stain can be observed. The blue staining is the result of the enzymatic digestion of α-1,6-glycosidic branches of the amylopectin which accounts for its reddish or purple color.

[0265] (b) Purification of a Debranching Enzyme from Solanum tuberosum and Detection of Peptide Sequences

[0266] Purification of a debranching enzyme from Solanum tuberosum and detection of peptide sequences was carried out according to known methods as follows:

[0267] For purification of a debranching enzyme from potato 500 g tuber tissue were homogenized in 1.5 l buffer B in a “Warring blender” for 1 min. The homogenate was strained through a sandwich of six layers of muslin cloth. If necessary, the pH was adjusted to a value of 5.8. The homogenate was subjected to centrifugation at 25,000×g for 30 min. Then proteins were precipitated from the supernatant by ammonium sulphate precipitation at 0° C. For this purpose, ammonium sulphate was continually added under stirring to the supernatant up to a concentration of 40% of the saturation concentration. When precipitation of proteins began, the mixture was stirred for another 30 min. After separation of the precipitated proteins by centrifugation at 25,000×g for 30 min the resulting supernatant was mixed with ammonium sulphate up to a concentration of 50% of the saturation concentration. Precipitation was carried on for another 30 min. Then the mixture was centrifuged at 25,000×g for 30 min and the precipitated proteins were resuspended in about 80 ml buffer B. After centrifugation at 30,000×g for 15 min the supernatant was removed and applied to an affinity column which had been equilibrated with buffer B. The material of the affinity column was expoxy-activated sepharose 6B (Sigma) to which β-cyclodextrin (Cycloheptaamylose; Sigma) had been coupled as described by Ludwig et al. (Plant Physiol. 74 (1984), 856-861). The affinity column was washed with buffer B until the eluate did not exhibit any absorption at 280 nm. A protein fraction having low affinity to the stationary phase was eluted using a β-cyclodextrin solution (1 mg/ml in buffer A) and was then discarded. At a β-cyclodextrin concentration of 10 mg/ml in buffer A the potato debranching enzyme was eluted (cf. FIG. 3).

[0268] The fraction of the eluate which was rich in debranching enzyme was subjected to electrophoresis in a denaturing PAAG according to the method of Laemmli (Nature 227 (1970), 680-685). The denatured protein was cut out from the gel and isolated. Peptide sequences were determined according to standard techniques. Peptide sequences of the debranching enzyme from Solanum tuberosum are shown in Seq ID No. 1 to Seq ID No. 12.

[0269] (c) Identification and Isolation of Genomic DNA Sequences Coding for a Debranching Enzyme of Solanum tuberosum by using Genetic Engineering

[0270] The peptide sequences as obtained according to section (b) were used to derive oligonucleotide sequences which represent regions of the gene coding for potato debranching enzyme when considering the degeneracy of the genetic code. Synthetic oligonucleotides were synthesized according to standard techniques in accordance with the derived oligonucleotide sequences. These synthetic oligonucleotides were used to screen genomic libraries.

[0271] First a genomic DNA library was constructed according to Liu et al. (Plant Mol. Biol. 17 (1991), 1139-1154). Subsequently, E. coli cells of the strain P2392 were infected with the phages containing the genomic DNA fragments and plated on a medium in Petri dishes in a density of about 30,000 per 75 cm2. The Petri dishes were incubated at 37° C. until the phage plaques had reached a suitable size (about 6-8 hrs). Then the Petri dishes were stored at 4° C. for several hours. Nitrocellulose membranes were placed on the lysed bacterial layer and were removed after one minute. The filters were incubated for 2 min in 0.2 M NaOH; 1.5 M NaCl, then for 2 min in 0.4 M Tris/HCl (pH 7.5) and finally for 2 min in 2×SSC. The filters were dried, UV cross-linked and incubated for 3 hrs in buffer H before the radioactively end-labelled oligonucleotides were added. After hybridization for 12 hrs the filters were washed two times for 15 min in 0.2×SSC/0.1% SDS and then subjected to autoradiography.

[0272] The temperature for hybridization and washing of the filters can be calculated as follows:

T+ 15=16.6×[Na+]+0.41×(% GColigonucleotide]+81.5−675/lengtholigonucleotide

[0273] The peptide sequences of the potato debranching enzyme depicted in Seq ID No. 1 or Seq ID No. 5 can be used to derive suitable oligonucleotide sequences. In order to achieve as high as possible hybridization temperature allowing sufficient specificity for hybrid formation, as long as possible oligonucleotides should be used. With increasing length, however, the degree of degeneracy also increases, i.e., the number of oligonucleotides having different sequence combinations. Degrees of degeneracy of up to 6,000 are acceptable. If several peptide sequences are known for a protein, corresponding oligonucleotides can be derived, combined and used as an oligonucleotide mixture for hybridization, thus increasing efficiency of hybridization.

[0274] For a part of the peptide sequence indicated in Seq ID No. 1 the sequence for an oligonucleotide probe of 20 bp length was derived. The oligonucleotide had a degree of degeneracy of 256 at a GC content of maximal 65% and minimal 40%, resulting in a maximum hybridization temperature of about 56° C. For a part of the peptide sequence indicated in Seq ID No. 5 the sequence for an oligonucleotide probe of 20 bp length was derived. The oligonucleotide had a degree of degeneracy of 384 at a GC content of maximal 55% and minimal 50%, resulting in a maximum hybridization temperature of about 60° C. Both oligonucleotides were used for hybridization as a mixture at a temperature of 54° C. The filters were washed at a temperature of 45° C.

[0275] The Sequence Template for the Synthesis of the Probes was as Follows:

Peptide 1:	NH2-Asp Ser Asp Asp Val Lys Pro Glu Gly-COOH	(Amino acids 8-16 in Seq ID No. 1)
mRNA:	5′ GAY GAY GUN AAR CCN CAR GG 3′
Probe 1:	3′ CTR CTR CAN TTY GGN CTY CC 5′	(Seq ID No. 19)
Peptide 5:	NH2-Ile Gln Val Gly Met Ala Ala-COOH	(Amino acids 3 bis 9 in Seq ID No. 5)
mRNA:	5′ AUH CAR GUN GUN AUG GCN CC 3′
Probe 2:	3′ TAD GTY CAI CCI TAC CGI CC 5′	(Seq ID No. 20)

[0276] In three screening cycles phage clones containing a DNA insert hybridizing to the probes used were individualized. In this manner about 40 plaques were identified when screening about 500,000 phage plaques.

[0277] These positive phage clones were used for hybridization to the cDNA sequence (Seq ID No. 17) isolated from spinach which codes for a debranching enzyme. In this way 3 phage clones could be isolated which also hybridize with the cDNA sequence from spinach. From one of the identified lambda phage clones, λDEpot, DNA was obtained according to standard techniques, the DNA insert was isolated and cleaved with the restriction endonuclease Sau3A. The resulting subfragments were ligated into a pBluescript vector which had been cleaved with BamHI. E. coli cells were transformed with the resulting plasmids. Transformed bacteria were plated on medium in Petri dishes. In order to determine which bacteria contain DNA inserts coding for debranching enzyme a colony hybridization was performed. For this purpose, a nitrocellulose membrane was placed on a solid medium in a Petri dish. Onto this membrane cells of E. coli colonies were transferred. The petri dish was incubated at 37° C. overnight and the E. coli cells on the membrane grew to colonies. The membrane was removed from the medium and incubated for 5 min in 10% SDS, 5 min in 0.5 M NaOH; 1.5 M NaCl, then for 5 min in 0.5 M Tris/HCl (pH 7.5) and subsequently for 5 min in 2×SSC. The filters were dried, UV cross-linked and incubated for 3 hrs in buffer H before radioactively end-labelled oligonucleotides were added. For hybridization the radioactively labelled probes 1 (Seq ID No. 19) and 2 (Seq ID No. 20) were used. After hybridization at 54° C. for 12 hrs the filters were washed at 45° C. two times for 15 min in 0.2×SSC/0.1% SDS and subjected to autoradiography. Bacteria of colonies hybridizing to the probe used were cultivated and plasmid DNA was isolated from the cells. In this manner various plasmids containing inserts that hybridized to the oligonucleotides used were isolated. Subsequently, part of the DNA sequence of the inserts of the isolated plasmids were determined according to the method of Sanger et al. (Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467).

EXAMPLE 6

[0278] Isolation of cDNA Sequences Coding for a Debranching Enzyme from Solanum tuberosum using Polymerase Chain Reaction

[0279] The partial genomic DNA sequences obtained in Example 5 were compared with the spinach cDNA sequence coding for the debranching enzyme. It was found that two of the identified inserts comprised DNA sequences that are homologous to two DNA sequences that are located in the spinach cDNA in a distance of about 500 bp from each other. Starting from these sequences two oligonucleotides were synthesized for PCR reaction. These two oligonucleotides had the following nucleotide sequences:

Oligonucleotide C
5′ AAGGTACCGG ATCCTCTGCT GATGGCAAGT GGACATTATT AGT 3′ (Seq ID No. 21)
Oligonucleotide D
5′ TTAAGCCCGG GCGATACGAC AAGGACCATT TGCATTACCA G 3′ (Seq ID No. 22)

[0280] Oligonucleotide C serves to introduce a BamHI restriction site at one end of the amplified DNA fragment. This oligonucleotide is partially homologous to the spinach cDNA in the range of nucleotides 1082 to 1110 of the DNA sequence depicted in Seq ID No. 17. Oligonucleotide D serves to introduce a SmaI restriction site at the other end of the fragment to be amplified. This oligonucleotide is homologous to the spinach cDNA in the range of nucleotides 1545 to 1571 of the DNA sequence depicted in Seq ID No. 17. These oligonucleotides were used for the amplification of a DNA fragment from a potato tuber cDNA library.

[0281] For this purpose, a cDNA library was established by preparing total RNA from the tuber tissue of potato according to Logemann et al. (Anal. Biochem. 163 (1987), 16-20). Polyadenylated mRNA was prepared from the total RNA according to standard techniques and was used for the synthesis of cDNA according to the method described by Gubler and Hoffmann (Gene 25 (1983), 263). The cDNA was ligated with commercially available EcoRI/NotI adapters, ligated into the EcoRI restriction site of the DNA of phage Lambda ZAP II (Stratagene) and packaged in phage heads.

[0282] From a cDNA library so constructed a DNA fragment of about 500 bp length was amplified by PCR using oligonucleotides C and D. This DNA fragment was cleaved with the restriction endonucleases BamHI and SmaI and ligated into a pBluescript vector which had been cleaved with BamHI and SmaI. The resulting plasmid was called pDBE-Pot (FIG. 4).

[0283] The isolated 500 bp fragment serves to isolate cDNA fragments comprising the entire coding region for potato debranching enzyme from a cDNA library in Lambda ZAP II constructed as described above by using conventional molecular genetic techniques.

EXAMPLE 7

[0284] Sequence Analysis of the cDNA Insert of Plasmid pDBE-Pot

[0285] From an E. coli clone obtained according to Example 6 plasmid pDBE-Pot (FIG. 4) was isolated and its cDNA insert determined according to the standard didesoxy method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467).

[0286] The insert has a length of 492 bp. The nucleotide sequence is indicated in Seq ID No. 23.

[0287] Analysis of the DNA sequence showed that the peptide sequences indicated in Seq ID No. 1 and in Seq ID No 2 are coded for by the DNA sequence indicated in Seq ID No. 23, with deviations at two positions each. Seq ID No. 1 corresponds to amino acids 6 to 26 of the amino acid sequence indicated in Seq ID No. 23 and Seq ID No. 2 corresponds to amino acids 80 to 99 of the amino acid sequence indicated in Seq ID No. 23. The deviations can be accounted for by the fact that the protein was isolated from potatoes of the variety Désirée, the cDNA library used, however, was constructed from potatoes of the variety Berolina.

EXAMPLE 8

[0288] Construction of Plasmid p35S-antiDBE-Pot, Transformation of Potato Plants as well as Characterization of the Starch Synthesized

[0289] A DNA fragment of about 500 bp length was isolated from plasmid pDBE-Pot by digestion with BamHI/SmaI, which fragment has the sequence indicated in Seq ID No. 23 and contains part of the coding region for potato debranching enzyme. This DNA fragment was cloned into vector pBinAR (Höfgen and Willmitzer, Plant Sci. 66 (1990), 221-230) which had been cleaved with BamHI/SmaI. Vector pBinAR is a derivative of binary vector Bin19 (Bevan, Nucleic Acids Res. 12 (1984), 8711-8721).

[0290] pBinAR was constructed as follows:

[0291] A fragment of 529 bp length comprising nucleotides 6909-7437 of the 35S promoter of the cauliflower mosaic virus (Franck et al., Cell 21 (1980), 285-294) was isolated as EcoRI/KpnI fragment from plasmid pDH51 (Pietrzak et al., Nucl. Acids Res. 14, 5857-5868) and ligated between the EcoRI and KpnI restriction sites of the polylinker of pBin19, resulting in plasmid pBin19-A.

[0292] A fragment of 192 bp length was isolated from plasmid pAGV40 (Herrera-Estrella et al., Nature 303, 209-213) using the restriction endonucleases PvuII and HindIII, which fragment comprises the polyadenylation signal of gene 3 of the T-DNA of Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3, 835-846) (nucleotides 11749-11939). After addition of SphI linkers to the PvuI restriction site the fragment was ligated into pBin19-A which had been cleaved with SphI and HindIII, resulting in pBinAR.

[0293] The resulting plasmid was called p35S-antiDBE-Pot and is depicted in FIG. 5.

[0294] Insertion of the cDNA fragment results in an expression cassette that is composed of fragments A, B and C as follows (FIG. 5):

[0295] Fragment A (529 bp) contains the 35S promoter of the cauliflower mosaic virus (CaMV). The fragment comprises nucleotides 6909 to 7437 of CaMV (Franck et al., Cell 21 (1980), 285-294).

[0296] Fragment B comprises part of the protein-encoding region of the cDNA coding for potato debranching enzyme. This part was isolated from pDBE-Pot as BamHI/SmaI fragment as described above and fused to the promoter in pBinAR in anti-sense orientation.

[0297] Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).

[0298] The size of the plasmid p35S-antiDBE-Pot is about 11.5 kb.

[0299] Vector p35S-antiDBE-Pot was transferred to potato plant cells via Agrobacterium tumefaciens-mediated transformation. Intact plants were regenerated from the transferred cells.

[0300] An analysis of the total RNA for the absence of endogenous mRNA coding for the debranching enzyme can be used to verify if the plants have been successfully genetically modified.

[0301] As a result of transformation, transgenic potato plants exhibited a reduced debranching enzyme activity (cf. FIG. 9).

[0302] In contrast to the starch granules from wild type plants which have a regular round shape the starch granules produced by transgenic plants have a rough, chapped or even frayed surface (see FIG. 10).

[0303] The starch synthesized by transgenic plants furthermore differs from that synthesized by wild type plants in its viscosity during gelation (“pasting”), in its gel stability and phosphate content.

[0304] Viscosity was determined with a Rapid Visco Analyser according to the method described above. The results are shown in FIG. 11. FIG. 11 in curve 4 shows a typical RVA curve for starch isolated from wild type plants of the potato variety Désirée. Curves 1 to 3 of the transformed plant lines have a considerably less marked viscosity maximum after heating to 96° C. as well as a higher increase in viscosity after cooling to 50° C., i.e., a higher final viscosity.

[0305] FIG. 12 shows the gel stability of gels prepared from starch of the inhibited plant lines as compared to gel from wild type starch. The gel stability of the modified starch is substantially different. The force that is necessary to deform the gel is substantially greater than the force that is necessary to deform a corresponding gel prepared from wild type starch.

[0306] The phosphate content of the starch synthesized by transgenic plants is—also dependent on the degree of antisense-inhibition—above the value for starch synthesized by wild type plants (see Table 2). The measuring inaccuracy is about ±5%.

[0307] The amylose content is calculated according to Hovenkamp-Hermelink et al. (Potato Res. 31 (1988), 241-246). Depending on the transgenic plant line the amylose content is approximately the same or slightly increased vis-á-vis the amylose content of wild type starch (see Table 2).

TABLE 2
Plants	nmol glucose-6-phosphate/mg starch	% amylose
Wild type	9.00	20.4
RE500-47	14.66	21.7
RE500-81	11.19	19.7
RE500-75	10.42	22.5

[0308]

Claims

1. A DNA molecule coding for a plant protein having the biological activity of a debranching enzyme, or a biologically active fragment thereof, selected from the group consisting of: (a) DNA molecules coding for a protein having the amino acid sequence indicated in Seq ID No. 18; (b) DNA molecules containing the nucleotide sequence indicated in Seq ID No. 17; (c) DNA molecules coding for a protein containing the amino acid sequence indicated in Seq ID No. 24; (d) DNA molecules containing the nucleotide sequence indicated in Seq ID No. 23; (e) DNA molecules the nucleotide sequence of which deviates from the nucleotide sequence of a DNA molecule according to (a), (b), (c) or (d) due to the degeneracy of the genetic code; and (f) DNA molecules that hybridize to a DNA molecule according to (a), (b), (c), (d) or (e).

2. The DNA molecule according to claim 1, wherein the protein having the enzymatic activity of a debranching enzyme exhibits at least one of the peptide sequences indicated in Seq ID No. 1 to Seq ID No. 14.

3. The DNA molecule according to claim 1 or 2, wherein the protein has a molecular weight of about 100±10 kD upon SDS gel electrophoresis.

4. The DNA molecule according to any of claims 1 to 3 which is derived from a taxonomically higher plant.

5. The DNA molecule according to claim 4 which is derived from a plant of the family Solanaceae.

6. The DNA molecule according to claim 5 which is derived from Solanum tuberosum.

7. The DNA molecule according to claim 4 which is derived from a plant of the family Chenopodiaceae.

8. The DNA molecule according to claim 7 which is derived from Spinacia oleracea.

9. A vector containing a DNA molecule according to any of claims 1 to 8.

10. The vector according to claim 9, wherein the DNA molecule is linked in sense orientation to regulatory DNA sequences which allow transcription and translation in procaryotic or eucaryotic cells.

11. A host cell which is transformed with a DNA molecule according to any of claims 1 to 8 or with a vector according to claim 9 or 10, or which is derived from such a cell.

12. A process for producing a plant protein having the biological activity of a debranching enzyme, or an active fragment thereof, wherein host cells according to claim 11 are cultivated under suitable conditions and the protein is obtained from the culture.

13. A protein having the biological activity of a debranching enzyme, or a biologically active fragment thereof (a) which is obtainable by a process according to claim 12; or (b) which is coded for by a DNA molecule according to any of claims 1 to 8, except for proteins from potato or spinach.

14. The host cell according to claim 11 which is a transgenic plant cell.

15. Transgenic plants containing transgenic plant cells according to claim 14.

16. Starch obtainable from plant cells according to claim 14 or from plants according to claim 15.

17. A transgenic plant cell the debranching enzyme activity of which is reduced as compared to that of non-transformed cells due to the inhibition of the transcription or translation of endogenous nucleic acid molecules coding for a debranching enzyme.

18. The transgenic plant cell according to claim 17 containing a DNA molecule according to any of claims 1 to 8 or a part of such a DNA molecule, with the DNA molecule or part thereof being linked in antisense orientation with regulatory DNA sequences allowing transcription in plant cells.

19. The transgenic plant cell according to claim 17 which expresses a ribozyme which specifically cleaves transcripts of DNA molecules coding for a debranching enzyme.

20. Transgenic plants containing plant cells according to any of claims 17 to 19.

21. Starch obtainable from plant cells according to any of claims 17 to 19 or from plants according to claim 20.

22. Propagating material of plants according to claim 15 or 20 containing plant cells according to claim 14 or any of claims 17 to 19.

23. Use of the starch according to claim 16 or 21 for the production of food or industrial products.