DNA sequence encoding glycerol 3-phosphate acyltransferase

Abstract

Disclosed is a DNA sequence which comprises a nucleotide sequence coding for a polypeptide having glycerol 3-phosphate acyltransferase activity and having the amino acid sequence corresponding substantially to the amino acid sequence from A to B represented in FIGS. 1(a) and (b). The DNA sequence has an ability to biotechnologically produce a glycerol 3-phosphate acyltransferase.

Description

BACKGROUND OF THE INVENTION

1. Field of the Art

The present invention relates to an isolated DNA sequence coding for a polypeptide which has enzymatic activity of glycerol 3-phosphate acyltransferase (referred to hereinafter as ATase) as is produced by Arabidopsis thaliana Heynhold.

2. Related Art

Lipid components of the plant biomembranes change from a liquid-crystalline phase to a gel phase when the environmental temperature decreases, which is accompanied by changes in the properties of these biomembranes. It is presumed that the biomembranes cannot fulfill its intact functions in the gel phase as they lose the selective permeability, which results in disfunctions of cells. Among the lipids, phosphatidylglycerol (referred to hereinafter as PG) changes from the liquid-crystalline phase to the gel phase at a high temperature and thus tends to gelate at a high temperature. Therefore, the temperature sensitivity of biomembranes greatly varies with the properties of PG. In this connection, the tendency of gelation of PG molecules depends on the composition of fatty acids that constitutes the PG. The transfer of an acyl-group to glycerol 3-phosphate (referred to hereinafter as G-3-P) is carried out by the ATase. That is, the transfer reaction of an acyl-group from an acyl-carrier protein (referred to hereinafter as ACP) to G-3-P is catalyzed by the ATase.

In plants, fatty acids are synthesized only in chloroplasts. Acyl-ACPs, the substrates of the ATase, are mainly palmitoyl-ACP (referred to hereinafter as 16:0-ACP) and oleoyl-ACP (referred to hereinafter as 18:1-ACP). The selection between these two substrates by the ATase depends on a property of the ATase itself, that is, substrate selectivity. The substrate selectivity of the ATase has been investigated for a variety of plants. For example, the ATase from chilling-resistant plants, spinach and pea, has a high substrate selectivity to the 18:1-ACP [Eur. J. Biochem., 129 (1983), 629-636], and the PG of these plants remains in liquid-crystalline phase even at a relatively low temperature. On the other hand, since the ATase from chilling-sensitive plants does not distinguish 18:1-ACP from 16:0-ACP and transfers both fatty acids in an almost equal proportion, the PG of squash gelates at a relatively high temperature (details will be described later).

Among the ATases of plants, that of squash, a chilling-sensitive plant, is the only one for which the complete amino acid sequence has been elucidated. The sequence has been applied for a patent as "a DNA sequence coding for glycerol 3-phosphate acyltranspherase" by the present inventor (Japanese Patent Laid-Open Publication No. 235594/1989). The complete amino acid sequence of an ATase from chilling-resistant plants is elucidated for the first time by the present invention in which the sequence is derived from the enzyme of Arabidopsis thaliana Heynhold. While the amino acid sequence of the ATase of Arabidopsis thaliana Heynhold is as a whole similar to that of squash, it has regions which are greatly different from that of squash. The difference in the substrate selectivity of the ATases is probably accounted for by this difference in the amino acid sequences.

It is indicated that a "transit peptide" is present at the N-terminal domain of proteins which are synthesized in cytoplasm and transferred to chloroplasts. It has been demonstrated that such a transit peptide is necessary for their transfer [Nature, 313 (1985), 358-363].

DISCLOSURE OF THE INVENTION

Outline of the Invention

The present invention provides an isolated DNA sequence coding for a polypeptide which has an enzymatic activity of those ATases that are useful for improving the properties of a membrane lipid, PG, in chloroplasts.

That is, the DNA sequence according to the present invention is characterized by that it includes a nucleotide sequence coding for a polypeptide which has a glycerol 3-phosphate acyltransferase activity and whose amino acid sequence corresponds substantially to the amino acid sequence from A to B shown in FIGS. 1(a) and (b).

Effect

The polypeptide or protein obtained by the expression of the DNA sequence according to the present invention has a glycerol 3-phosphate acyltransferase activity and exhibits a high substrate selectivity to 18:1-ACP (see Experimental Example 3 below).

Therefore, it is postulated that by the introduction and expression of the DNA sequence according to the present invention into a chilling-sensitive plant (containing a relatively large portion of saturated molecular species of PG), it will be possible to increase the content of unsaturated molecular species of the PG, that is, to make the content close to that in the fatty acid composition of chilling resistant plants. Techniques for introducing and expressing a DNA sequence in plants are general techniques which have already been practiced for many kinds of plants such as tobacco, petunia, potato and the like.

Further, it is postulated that the use of the DNA sequence obtained in the present invention as a probe facilitates to clone genes encoding the enzymes from chilling-resistant plants, which have a similar substrate selectivity, by the methods which are usually used in the field of genetic engineering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-(b) show an example of the amino acid sequence of a peptide containing as a portion the ATase which is an object of the present invention, and the nucleotide sequence coding for this amino acid sequence;

FIGS. 2(a)-(d) show the nucleotide sequence of genomic DNA containing the ATase gene of Arabidopsis thaliana Heynhold and obtained in Experimental Example 1 of the present invention, along with the amino acids encoded by the exon regions; and

FIGS. 3(a)-(b) show the nucleotide sequence of cDNA containing the ATase gene of Arabidopsis thaliana Heynhold as a portion and obtained in Experimental Example 2 of the present invention, along with the amino acid sequence encoded.

DETAILED DESCRIPTION OF THE INVENTION

The ATase gene

Definition

The DNA sequence having an ability to biotechnologically produce the ATase according to the present invention, that is the ATase gene, codes for a polypeptide which has an ATase activity and whose amino acid sequence corresponds substantially to the amino acid sequence from A to B shown in FIGS. 1(a)-(b). The term "DNA sequence" herein means a polydeoxyribonucleotide chain having a certain length In the present invention, the "DNA sequence" is specified by the amino acid sequence of the polypeptide encoded by the DNA sequence, said amino acid sequence including modified sequences which will be described hereinafter. Since the polypeptide has a definite length, the "DNA sequence", which includes degenerated isomers which will be described hereinafter, has a definite length The DNA sequence of the present invention contains the gene encoding the ATase and is useful for the biotechnological production of this polypeptide. However, the biotechnological production of this polypeptide cannot be conducted only by the DNA sequence having a definite length defined by the length of the ATase; the presence of DNA sequences with an appropriate length are necessary at the 5'-upstream and 3'-downstream. The biotechnological production of the polypeptide is also possible when the DNA sequence encoding this polypeptide comprises the coding region in exons which are interrupted by several introns, and it is reported that the introns sometimes increase the level of gene expression (The abstracts of the 12th Annual Meeting of Molecular Biological Society, Japan, p. 240, 3J-23, 1989). As described above, a transit peptide of required at the N-terminus of the mature ATase for its transfer from cytoplasm to chloroplasts. Therefore, if the ATase synthesized in cytoplasm is required to be transferred to chloroplasts, the gene encoding the ATase needs to have a DNA sequence coding for a transmit peptide of the 5'-upstream.

Accordingly, the term "DNA sequence" in the present invention includes, in addition to the DNA sequence having a specific length [defined by the length of A to B of the corresponding polypeptide shown in FIGS. 1(a) and (b)], linear or circular DNA strand containing this DNA sequence of the specific length.

Typical existing forms of the DNA sequence according to the present invention are the DNA sequence inserted in plasmid or phase DNAs and the DNA sequence inserted in either plasmid, phage or genomic DNAs carried by either microorganisms (particularly bacteria), phage particles or plants. The bacteria herein include Escherichia coli and Agrobacterium tumefaciens.

A preferred existing form of the DNA sequence according to the present invention is the DNA sequence inserted into a plant genome, where it is aligned with other DNA sequences as follows in order to stably express the ATase: (in this order) a promoter, a DNA sequence of a regulatory region for translation, a DNA sequence coding for a transit peptide to chloroplasts, the DNA sequence according to the present invention, a DNA sequence coding for a termination codon, a DNA sequence coding for a poly(A)addition signal and a terminator. As the promoter, the DNA sequence of a regulatory region for translation, the DNA sequence coding for a termination codon, the DNA sequence coding for a poly(A)addition signal and the terminator, an appropriate combination of those which are well-known in the field can be used. The DNA sequence coding for a transit peptide to chloroplasts may be that of the ATase gene obtained in the present invention [nucleotide number 190-270 or 1-270 in FIG. 1(a)], or a well-known sequence such as that of the genes for the small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase of spinach.

Polypeptide encoded by the gene

As described above, the DNA sequence according to the present invention is specified by the polypeptide encoded. The polypeptide has an ATase activity, and its amino acid sequence corresponds substantially to the amino acid sequence from A to B shown in FIGS. 1(a) and (b). It is needless to say that FIG. 1(b) represents the continuation of FIG. 1(a). The phraseology "the amino acid sequence corresponds substantially to the amino acid sequence from A to B shown in FIGS. 1(a) and (b)" herein means that the polypeptide may have deletions, substitutions, additions or the like in some of the amino acids as far as the polypeptide possesses the ATase activity.

The part from C to A (or from C' to A) of the amino acid sequence represented in FIGS. 1(a) and (b) appears to be at least a part of the transit peptide by comparison with the ATase of squash. This part shall also include a variety of modifications of the amino acid sequence as described above.

The typical polypeptide having an ATase activity in the present invention corresponds to the amino acid sequence from A to B (or C to B, or C' to B) in FIGS. 1(a) and (b) and consists of 369 (or 396 or 459 ) amino acids, and such an amino acid sequence has not hitherto been reported.

The ATase as the object of the present invention is the enzyme specified as EC 2. 3. 1.15.

The Nucleotide sequence

The DNA sequence coding for the ATase includes the one which has the nucleotide sequence represented by A to B (or C to B, or C' to B) in FIG. 1 or degenerate isomers thereof as well as the ones which have the nucleotide sequence corresponding to the variations of the amino acid sequence of the ATase as described above or degenerate isomers thereof. The term "degenerate isomer" herein means a DNA sequence which is different from others only in the degenerate codons and codes for the same polypeptide. For instance, a DNA sequence having the nucleotide sequence from A to B (from C to B, or from C' to B) shown in FIG. 1 but a codon substitution of, for example, an AAC (corresponding to Asn) with an AAT, which is in the degenerate relationship with AAC, shall be referred to as a degenerate isomer in the present invention.

A preferred embodiment of the DNA sequence according to the present invention has at least one termination codon such as TAG next to the 3'-terminus. The 5'-upstream and/or 3'-downstream of the DNA sequence of the present invention may also be followed by DNA sequences (sequence) of a certain length as an untranslated regions (region).

Another preferred embodiment is a DNA sequence which includes the nucleotide sequence from A to B (or C to B, or C' to B) shown in FIG. 1 as the coding region in exons interrupted with one or more introns. This preferred embodiment is illustrated by D to E (or F to E, or F' to E) in FIG. 2. As the introns, well-known introns can be used appropriately in addition to those illustrated in FIG. 2 (see the abstract of the 12th Annual Meeting of Molecular Biological Society (described above), Japan).

The nucleotide sequence shown in FIGS. 2(a)-(d) was determined by the dideoxy termination method from the genomic DNA encoding the ATase obtained from the leaves and stems of seedlings of Arabidopsis thaliana Heynhold.

The nucleotide sequence shown in FIGS. 3(a)-(b) was determined by the dideoxy termination method from the cDNA coding for the ATase of Arabidopsis thaliana Heynhold.

Acquirement of the DNA sequence

One of the methods for acquiring a DNA sequence described above encoding the amino acid sequence of the ATase comprises the chemical synthesis of at least a part of the DNA sequence.

If one considers that the ATase has at least 369 amino acid residues, it is more preferable than the chemical synthesis to acquire the DNA sequence by genetic engineering processes. These processes comprise first constructing a cDNA library from mRNAs derived from the leaves and/or stems of Arabidopsis thaliana Heynhold by methods usually used in the field of genetic engineering such as the OKAYAMA-BERG method [Mol. Cell Biol., 2 (1982), 161-170]and then isolating the DNA sequence by widely used screening methods such as immunological screening and nucleic acid hybridization using appropriate probes.

In this connection, the present inventor had isolated the cDNA for the ATase derived from Cucurbita moschata Duch (see Japanese Patent Laid-Open Publication No. 235594/1989). The inventor then constructed a genomic DNA library derived from leaves and stems of Arabidopsis thaliana Heynhold seedlings using a .lambda. phage vector which is usually used in the field of genetic engineering. By screening the library by the plaque hybridization method using the cDNA for the squash ATase labeled with a 32P radio isotope as a probe, the inventor obtained a genomic DNA sequence containing the DNA sequence of the present invention (see Experimental Example 1 below for details). Further, as an alternative to this, the present inventor obtained a cDNA clone containing the DNA sequence of the present invention from a cDNA library of Arabidopsis thaliana Heynhold using this genomic DNA as a probe (see Experimental Example 2 below for details).

Arabidopsis thaliana Heynhold, the source of the DNA sequence of the present invention, is a plant which grows naturally in Japan and is described in "MAKINO SHINSHOKUBUTSU ZUKAN", (Hokuryu-kan, 1982, p. 218). Its seeds are available from Arabidopsis Information Service (Prof. Dr. A. R. Kranz, Botanisches Institut, J. W. Goethe Universitaet, Siesmayerstr., 70, Postfach 111 932, D-6000 Frankfurt am Main 11, Federal Republic of Germany).

EXPERIMENTAL EXAMPLES

Experimental Example 1

Isolation of the ATase gene from a genomic DNA library of Arabidopsis thaliana Heynhold

(1) Construction of a qenomic DNA library

(i) Preparation of genomic DNA

Genomic DNA was obtained from about 10 g of leaves and stems of Arabidopsis thaliana Heynhold (Lansberg strain) as described in Current Protocols in Molecular Biology, Vol. 1, pp. 2,3,1-2,3,3 (Edited by F. M. Ausbel, R. Bront, R. E. Kingston, D. D. Moore, J. G. Seidman, J. A. Smith and K. Struhl, John Wiley and Sons, 1987).

(ii) Construction of genomic DNA library

The genomic DNA was partially digested with a restriction enzyme Sau3AI, inserted into the BamHI site of a lambda phage vector .lambda.DASH (Stratagene) and packaged in vitro using an in vitro packaging kit (GIGAPACK GOLD; Stratagene) to give a genomic DNA library in .lambda. phage.

(2) Isolation of the ATase gene

(i) Screening of clones retaining the ATase gene

Escherichia coli strain P2392 [Stratagene) was infected with the phage library, and three plates (10 cm .times.14 cm) with 6.times.10.sup.3 -6.times.10.sup.4 plaques each were screened. The phages were transferred to filters, which were incubated at 68.degree. C. for 2 hours in a hybridization solution containing 5 .times.Denhart's solution (0.1% Ficoll, 0.1% polyvinylpyrrolidone, 0.1% bovine serum albumin), 6.times.SSC (900 mM NaCl, 90 mM trisodium citrate, pH 7.4), 10% dextran sulfate, 0.1% sodium dodecylsulfate and 100 .mu.g/ml of salmon sperm DNA.

A cDNA fragment for the ATase of squash (Cucurbita moschata Duch) was obtained by excising, with a restriction enzyme EcoRI, from the recombinant plasmid pAT 03 carrying the cDNA for the squash ATase from Escherichia coli AT-03 [Deposit No.: FERM BP-3094 (Original Deposit No.: FERM P-9934); Deposit Date: Mar. 11, 1988; Indication for distinguishment: AT-03]. This cDNA fragment was subjected to nick translation (nick translation kit; Takara Shuzo) with .sup.32 P-dATP to give a probe with a specific activity of about 10.sup.8 dpm/.mu.g.

The probe was added to the hybridization solution and the filters were further incubated in this solution at 50.degree. C. for 12 hours. Then, the filters were washed at 40.degree. C. with 2.times.SSC and 0.1% sodium dodecylsulfate solution and subjected to autoradiography to select phages which hybridized strongly to the probe.

The genomic DNA of Arabidopsis thaliana Heynhold was excised from the phage DNA with restriction enzymes BamHI and XbaI and subjected to 0.8% agarose gel electrophoresis to recover a 3.6 kbp DNA fragment (referred to hereinafter as BX3.6). This fragment strongly hybridized to the probe. It was subcloned to a plasmid vector pBLUESCRIPT (Stratagene) to give a plasmid pBX3.6. Similarly, a 2.6 kbp DNA fragment (referred to hereinafter as BB2.6) was excised from the phage DNA with a restriction enzyme BamHI and was subcloned to give a plasmid pBB2.6. The nucleotide sequences of the genomic DNA fragments of Arabidopsis thaliana Heynhold in pBX3.6 and pBB2.6 were analyzed by the dideoxy termination method (Proc. Natl. Acad. Sci., USA, 84 (1987), 4767-4771). The DNA sequence with a total length of 3,397 bp which encompassed BX3.6 and BB2.6 contained several open reading frames (from F to E or from F' to E in FIG. 2) (in which the nucleotide Nos. 1-1526 and 1527-3397 are derived from BX3.6 and BB2.6, respectively). By the comparison of the nucleotide sequence of the open reading frames with that of the cDNA for the squash ATase, and by the search for the conserved exon-intron junction sequence [Nucl. Acids Res., 16 (1988), 7159-7176], a coding region consisting of 1,188 bp or 1,377 bp was found in exons. This is shown in FIG. 1. In consideration that this coding region in exons is highly homologous to the cDNA for the squash ATase in both the nucleotide and the amino acid sequences, it was deduced that the DNA sequence represented from C to B (or from C' to B) in FIG. 1 encodes the precursor of the ATase containing a transit peptide to chloroplasts, consisting of 396 (or 459) amino acids with a molecular mass of 43,955 (or 50,431). It was also found that DNA sequences consisting of 587 bp (or 398 bp) and 210 bp were present at the upstream (nucleotide Nos. 1-587 (or 1-398) in FIG. 2) and downstream (nucleotide Nos. 3188-3397 in FIG. 2) of the coding region, respectively. It is presumed that the upstream and/or downstream region contains at least a part of the regulatory regions for translation. In particular, a termination codon (which is the DNA strand of nucleotide Nos. 3188-3190 in FIG. 2) was present in the downstream region.

It is assumed that the amino acid sequences C - A (or C'- A) in FIG. 1(a) or F - D (or F'- D) in FIG. 2(a) is a transit peptide to chloroplasts by comparison with the squash ATase.

Experimental Example 2

Isolation of the ATase cDNA from a cDNA library of Arabidopsis thaliana Heynhold

(1) Construction of a cDNA library

(i) Preparation of RNA

Total RNA was prepared from about 15 g of leaves and stems of Arabidopsis thaliana Heynhold (Lansberg strain) according to the method described in Current Protocols in Molecular Biology, Vol. 1, pp. 4.3.1-4.3.4 (Edited by F. M. Ausbel, R. Bront, R. E. Kingston, D. D. Moore, J. G. Seidman, J. A. Smith and K. Struhl, John Wiley and Sons, 1987). Poly(A)+RNA was prepared from the total RNA according to Wolf et al. (Nucl. Acids Res., 15 (1987), 2911-2926).

(ii) Construction of a cDNA library

DNA complementary to the above poly(A).sup.+ RNA was synthesized according to the manual of the cDNA synthesis kit available from Pharmacia, using an oligo(dT) nucleotide as a primer. An EcoRI adapter containing a NotI recognition sequence (Pharmacia) was ligated at each terminus of the double strand cDNA thus synthesized, which was followed by ligation to the EcoRI site of a .lambda. phage vector .lambda.ZAPII (Stratagene). The phage DNA was packaged in vitro using an in vitro packaging kit (GIGAPACK II GOLD: Stratagene) to give a cDNA library in .lambda.ZAPII.

(2) Isolation of the ATase cDNA

(i) Screening of clones retaining the ATase cDNA

Escherichia coli strain XLl-Blue (Stratagene) was infected by the .lambda. phage library, and five plates (10 cm .times.14 cm) with 2.times.10.sup.4 plaques each were screened. The phages were transferred to filters, which were incubated at 65.degree. C. for 1 hour in a hybridization solution containing 6 .times.SSC (900 mM NaCl, 90 mM sodium citrate, pH 7.4), 0.05% skim milk and 0.02% sodium azide. A fragment of the genomic ATase gene of Arabidopsis thaliana Heynhold was obtained by excising, with a restriction enzyme BamHI, from the recombinant plasmid pBB2.6 carrying a fragment of the ATase gene of Arabidopsis thaliana Heynhold [Experimental Example 1(2)(i)]. This DNA fragment was subjected to nick translation (nick translation kit, Takara Shuzo) with .sup.32 P-dATP to give a probe with a specific activity of about 10.sup.7 dpm/.mu.g.

The probe was added to the hybridization solution and the filters were further incubated in this solution at 65.degree. C. for 16 hours. Then, the filters were washed at 65.degree. C. with 1 .times.SSC and 0.1% sodium dodecylsulfate solution and subjected to autoradiography to select phages which hybridized strongly to the probe.

(ii) Structural analysis of the ATase cDNA

Inserts were excised from the phage DNAs with a restriction enzyme EcoRI and subjected to 1% agarose gel electrophoresis to determine the size of the fragments. It was found that one of the DNA fragments was about 1.4 kbp. This fragment was subcloned in a plasmid vector pBLUESCRIPT (Stratagene) to give a plasmid pARAT. The nucleotide sequence of the fragment was determined by the dideoxy termination method (Proc. Natl. Acad. Sci., USA, 84 (1987), 4767-4771) from pARAT.

As a result, it was found that the insert had a length of 1,445 bp which contains an open reading frame consisting of 1,188 bp or 1,377 bp, which is shown in FIG. 3. In consideration of a high homology of the open reading frame to the cDNA for the squash ATase in both the nucleotide and the amino acid sequences, it was deduced that the DNA sequence represented from I to H (or from I' to H) in FIGS. 3 (a)-(b) encodes the precursor of the ATase containing a transit peptide to chloroplasts, consisting of 396 (or 459) amino acids with a molecular mass of 43,955 (or 50,431). This sequence coincides completely with the coding region in exons deduced from the genomic DNA sequence represented by C to B (or C' to B) in FIGS. 1(a) and (b). It was also found that non-coding regions of 204 bp (or 15 bp) and 53 bp were present at the upstream (nucleotide Nos. 1-204 (or 1-15 in FIG. 3(a)) and downstream (nucleotide Nos. 1393-1445 in FIG. 3(b)) of the open reading frame, respectively. It is presumed that at least a part of the regulatory regions for translation is present in these upstream and/or downstream region. The open reading frame had a termination codon (which corresponds to the DNA sequence of nucleotide Nos. 1393-1395 in FIG. 3(b)) at its 3'-terminus. It is likely that the amino acid sequence I - G (or I'- G) in FIG. 3(a) is a transit peptide to chloroplasts by comparison with the squash ATase.

Experimental Example 3

Expression of the ATase gene of Arabidopsis thaliana Heynhold and confirmation of enzyme activity (Expression of the ATase genes of Arabidopsis thaliana Heynhold and squash (control) in E. coli and the comparison of their substrate selectivities)

(1) Insertion of cDNAs for the ATases of Arabidopsis thaliana Heynhold and squash into an expression-type vector plasmid in Escherichia coli

(i) Construction of expression vector plasmids

a. Ligation of the ATase cDNA of Arabidopsis thaliana Heynhold with pET3c

The plasmid pARAT obtained in Experimental Example 2(2) (ii) was digested with restriction enzymes HgaI (the restriction site is shown in FIG. 3(a)) and EcoRI (the restriction site is in the vector sequence downstream of H in FIG. 3(b)), and a 1.1 kbp fragment of the ATase cDNA of Arabidopsis thaliana Heynhold was isolated from a low melting agarose gel. The fragment was made blunt-ended with the Klenow fragment Meanwhile, plasmid pET3c (Novagen) was digested with a restriction enzyme BamHI and made blunt-ended with the Klenow fragment, and then the phosphoryl group at the 5'-terminus was removed with bacterial alkaline phosphatase. The cDNA fragment of the ATase of Arabidopsis thaliana Heynhold and pET3c thus obtained were ligated by T4DNA ligase to give an expression vector plasmid pAR1 containing T7 promoter, T7 leader sequence, the ATase cDNA of Arabidopsis thaliana Heynhold (containing the nucleotide sequence G - H in FIGS. 3(a) and (b), that is, A - B in FIGS. 1(a) and (b)), and T7 terminator.

b. Liqation of the ATase cDNA of squash with pET3a

Plasmid pAT-03 containing the cDNA for the squash ATase was prepared from E. coli AT-03 (FERM BP-3094), digested with restriction enzymes EcoRI and NaeI and then subjected to an electrophoresis on a low melting agarose gel to isolate a 1.2 kbp cDNA fragment of the squash ATase. Then this fragment was made blunt-ended with the Klenow fragment. Meanwhile, plasmid pET3a was digested with a restriction enzyme BamHI and was made blunt-ended with the Klenow fragment. Then the phosphoryl group at the 5'-terminus was removed with bacterial alkaline phosphatase. The cDNA fragment of the squash ATase and pET3a thus obtained were ligated by T4DNA ligase to give an expression plasmid pSQ1 containing T7 promoter, T7 leader sequence, the ATase cDNA of squash and T7 terminator.

(2) Introduction of pAR1 and pSQ1 into E. coli BL21 (DE3) containing the gene for T7 RNA polymerase on the genome

Competent cells of Escherichia coli BL21 (DE3) (Novagen) were prepared as described in Molecular Cloning, pp. 250-251 (Edited by T. Maniatis, E. F. Fritsch and J. Sambrook, 1982). Either of the plasmid pARl or pSQ1obtained as above (1) was introduced into the competent cells, and selection with ampicillin gave a transformant BLAR1or BLSQ1, respectively. (3) Expression and preparation of the ATases of Arabidopsis thaliana Heynhold and squash

The transformant BLAR1 or BLSQ1 obtained as above (2) was inoculated in to 500 ml of the LB medium (containing 200 .mu.g/ml of ampicillin) and cultured at 37.degree. C. Cells were grown until the turbidity of the culture reached 0.5 O. D. at a wavelength of 600 nm. Then isopropyl-thio-galactoside was added so that the final concentration was 0.4 mM, and the culture continued for 3 hours to induce the expression of the ATase protein. Bacterial cells were collected from the culture by centrifugation at 14,000 .times.g for 10 minutes. The pellets were rinsed with 50 mM Tris-HCl (pH 7.4) and resuspended in HM buffer (45 mM Tris-HCl, pH 7.4, 2 mM DTT, 10% glycerol, 10 mM sodium ascorbate, 1 mM benzamidine-HCl, 10 .mu.g/ml leupeptin, 5 mM 6-aminohexanoic acid). The bacterial suspension was passed through a French pressure cell at 10,000 psi to break the cells. The homogenate was centrifuged at 16,000 .times.g for 10 minutes and further at 100,000 .times.g for 60 minutes, and the supernatant was recovered as a crude enzyme fraction. The crude enzyme fraction was subjected to SDS electrophoresis on a 10% polyacrylamide gel and stained with Coomassie-Brilliant Blue to detect the ATase of Arabidopsis thaliana Heynhold or squash as a protein having a relative molecular mass of about 40 kDa.

(4) Assay of the ATase activity

The ATase activity of the crude enzyme fractions prepared as in (3) was assayed by the method of Nishida et al. (Plant Cell Physiol., 28 (1987), 1071-1079) using 16:0-CoA and L-[U-.sup.14 C] glycerol 3-phosphate as substrates. Both of the crude enzyme fractions from the E. coli transformants BLAR1 and BLSQ1 exhibited the ATase activity (the transfer of 16:0 to glycerol 3-phosphate). The specific activities of ATase in the fractions were 2,000 and 530 nmol/min.mg protein, respectively.

(5) Substrate selectivity of the ATase activity of the expressed enzymes

The substrate selectivity of the ATase activity thus obtained was analyzed according to Frentzen et al. (Plant Cell Physiol., 28 (1987), 1195-1201). The reaction mixture contained 30 mM of glycerol 3-phosphate, 1.5 .mu.M each of [1-.sup.14 C]16:0-ACP and [1-.sup.14 C]18:0-ACP, and the crude enzyme fraction of the expressed ATase corresponding to the enzyme activity of about 180 pmol/min. The selectivity was assayed at pH 7.4 and 8.2. The results showed that the expressed ATases of Arabidopsis thaliana Heynhold and squash had different substrate selectivity. This difference was unexpected from the homology of the nucleotide sequences of their cDNAs. That is, the ATase of Arabidopsis thaliana Heynhold has a substrate selectivity to 18:1-ACP higher than the ATase of squash (see table 1). These results suggested that the expression of the ATase gene of the present invention (ATase gene of Arabidopsis thaliana Heynhold) in a chilling-sensitive plant could increase the content of unsaturated fatty acids in PG, which is a compound of the chloroplast membrane lipids, and as a result could change the composition of fatty acids in PG to a chilling-resistant type.

TABLE 1______________________________________Substrate selectivity of the glycerol-3-Pacyltransferase activities expressed in solublefractions of E. coli Incorporation into lysophosphatidic acid.sup.*1cDNA source 18:1/16:0______________________________________ (pH) 7.4 8.2Arabidopsis (present invention) 73:27.sup.*2 65:35.sup.*2Squash (control) 68:32.sup.*2 56:44.sup.*2______________________________________ .sup.*1 In the presence of 30 mM glycerol 3P and 1.5 .mu.M [1.sup.14 C] 18:1-ACP and 1.5 .mu.M [1.sup.14 C] 16:0-CP; .sup.*2 The ratio of the incorporation of 18:1 and 16:0.

Claims

1. An isolated DNA fragment encoding a polypeptide having glycerol 3-phosphate acyltransferase activity, said polypeptide having an amino acid sequence that corresponds substantially to the amino acid sequence from A to B shown in FIGS. 1(a) and (b), and said glycerol 3phosphate acyltransferase activity having a higher substrate selectivity to oleoyl-(acyl-carrier protein) than to palmitoyl (acyl-carrier protein).

2. An isolated DNA fragment according to claim 1, wherein the nucleotide sequence coding for the polypeptide corresponds to the nucleotide sequence from A to B represented in FIGS. 1(a) and (b) or degenerate isomers thereof.