Production of polyalkanoate

Abstract

Genes encoding polyhydroxyalkanoate synthase, β-ketothiolase and acetoacetyl CoA reductase are isolated from the publicly available bacterium Chromatium vinosum. Recombinant genomes of plants or other species of bacteria which contain these genes are capable of producing polyalkanoate polymers. The nucleotide sequences of the said three genes have been determined.

Description

[0001] This invention relates to the production of polyhydroxyalkanoate by the culture of microorganisms which produce same.

[0002] Poly-3-hydroxybutyrate is a linear polyester of D(−)-3-hydroxybutyrate. It was first dicovered in Bacillus megaterium in 1925. Polyhydroxy-butyrate accumulates in intracellular granules of a wide variety of bacteria. The granules appear to be membrane bound and can be stained with Sudan Black dye. The polymer is produced under conditions of nutrient limitation and acts as a reserve of carbon and energy. The molecular weight of the polyhydroxybutyrate varies from around 50,000 to greater than 1,000,000, depending on the microorganisms involved, the conditions o f growth, and the method employed for extraction of the polyhydroxybutyrate. Polyhydroxybutyrate is an ideal carbon reserve as it exists in the cell in a highly reduced state, (it is virtually insoluble), and exerts negligible osmotic pressure.

[0003] Polyhydroxybutyrate and related poly-hydroxyalkanoates, such as poly-3-hydroxyvalerate and poly-3-hydroxyoctanoate, are biodegradable thermo-plastics of considerable commercial importance.

[0004] The term “polyhydroxyalkanoate” as used hereinafter includes copolymers of polyhydroxybutyrate with other polyhydroxyalkanoates such as poly-3-hydroxyvalerate.

[0005] Polyhydroxyalkanoate is biodegradable and is broken down rapidly by soil microorganisms. It is thermoplastic (it melts at 180° C.) and can readily be moulded into diverse forms using technology well-established for the other thermoplastics materials such as high-density polyethylene which melts at around the same temperature (190° C.). The material is ideal for the production of biodegradable packaging which will degrade in landfill sites and sewage farms. The polymer is biocompatible, as well as biodegradable, and is well tolerated by the mammalian, including human, body, its degradation product, 3-hydroxybutyrate, is a normal mammalian metabolite. However, polyhydroxyalkanoate degrades only slowly in the body and its medical uses are limited to those applications where long term degradation is required.

[0006] Polyhydroxyalkanoate, produced by the microorganism Alcaligenes eutrophus, is manufactured, as a copolymer with of polyhydroxybutyrate and polhydroxyvalerate, by Imperial Chemical Industries PLC and sold under the Trade Mark BIOPOL. It is normally supplied in the form of pellets for thermoprocessing. However, polyhydroxyalkanoate is more expensive to manufacture by existing methods than, say, polyethylene. It is, therefore, desirable that new, more economic production of polyhydroxyalkanoate be provided.

[0007] An object of the present invention is to provide materials and a method for the efficient production of polyhydroxyalkanoate.

[0008] According to the present invention there is provided gene fragments isolated from the bacterium Chromatium vinosum and encoding PHA, polymerase, acetoacetyl CoA reductase and β-ketothiolase.

[0009] Preferably the C. vinosum is of the strain designated D, available to the public from the Deutsche Sammlung für Mikroorganismen under the Accession Number 180.

[0010] The invention also provides a 16.5 kb EcoRl fragment of C. vinosum DNA, designated PP10, hybridizable to a 5.2 kb SmaI/EcoRl fragment, designated SE52 isolated from Alcaligenes eutrophus and known to contain all three of said genes responsible for expression of PHAs.

[0011] The invention further provides a fragment of the said PP10 fragment, designated SE45, encoding the PHA-synthase and β-ketothiolase genes and a region, designated SB24, encoding the acetoacetyl CoA reductase gene.

[0012] The invention also provides a recombinant genome of a microorganism, preferably a bacterium or a plant, which contains one or more of said fragments designated PP10, SE45 and region SB24.

[0013] Finally, the invention provides a method for the manufacture of PHAS, comprising culturing the microorganism Chromatium vinosum, or a bacterium of a different species having stably incorporated within its genome by transformation one or more PHA synthesising genes from Chromatium vinosum.

[0014] The biosynthesis of polyhydroxyalkanoate from the substrate, acetyl-CoA involves three enzyme-catalysed steps.

[0015] The three enzymes involved are β-ketothiolase, acetoactyl-CoA-reductase and polyhydroxybutyrate-synthase, the genes for which have been cloned from Chromatium vinosum. The three genes are known to facilitate production of polyhydroxyalkanoates, the reactions involved being represented as follows: 1

[0016] The invention will now be described with reference to the accompnaying drawings, of which;

[0017] FIG. 1 is the physical map of the 16.5 kb EcoRl fragment of Chromatium vinosum DNA, designated PP10. The positions of the restriction sites and positions and names of the sub-fragments are shown. PHA-synthase and β-ketothiolase genes are located in fragment SE45 and acetoacetyl CoA reductase in region SB24;

[0018] FIG. 2 is the map of PP10 showing the positions of the β-ketothiolase and acetoacetyl CoA reductase genes and of the PHA-synthase gene open reading frames ORF2 and ORF3.

[0019] FIG. 3 is the complete nucleotide sequence of fragment SE45. The transcriptional start sites and terminators for the β-ketothiolase gene and for ORF3 and ORF 3 are shown. The positions of the “−10” and “−35” sequences are also shown, as are the positions of the putative ribosome binding sites (“s/d”). Translational start and stop (*) codon are also marked and the amino acid sequences of the β-ketothiolase, ORF2 and ORF3 are given.

[0020] FIG. 4 shows the alignment of the amino acid sequences of Chromatium vinosum ORF3 with PHA polymerase of Alcaligenes eutrophus and PHA polymerases 1 and 2 of Pseudomonas oleovorans.

[0021] FIG. 5 shows the complete nucleotide sequence of the DNA encoding PHA synthesis genes from Chromatium vinosum. The positions of PHA polymerase (phbC), acetoacetyl CoA reductase (phbB) and ketothiolase (phbA) genes are shown, and also the positions of ORF2, ORF4, ORF5 and ORF7.

[0022] FIG. 6 shows the alignment of the amino acid sequences of ketothiolases encoded by C. vinsoum (C.v.), A. eutrophus (A.e.), Zoogloea ramigera (Z.r.), Escherichia coli (E.c.), Saccharomyces uvarum (S.u.) and Rattus norvegicus (R.n.)

[0023] FIG. 7 shows the alignment of the amino acid sequences of acetoacetyl CoA reductases encoded by C. vinosum (C.v.), A. eutrophus (A.e.) and Z. ramigera (Z.r.)

[0024] FIG. 8 is a Table (Table 1) showing the heterologous expression in Escherichia coli of DNA fragments from C. vinosum. Activities of PHA biosynthetic enzymes expressed by the different fragments are shown. The levels of PHA accumulated in E. coli transformed with the fragments are also given.

EXAMPLE

[0025] The organism C. vinosum was a gift from Dr J. Imhoff of the University of Bonn, Germany.

1. Isolation of DNA Fragments from C. vinosum Encoding PHA Synthesis Genes

[0026] A 5.2 kb SmaI/EcoRl fragment (SE52), which codes for all three PHA biosynthetic genes has previously been isolated from Alcaligenes eutrophus [Schubert et al., J. Bacteriol. 170 (1988)]. This fragment was used to detect PHA biosynthetic genes of C. vinosum. EcoRl restricted genomic DNA of C. vinosum was blotted on to a nylon membrane and hybridized with biotinylated SE52 DNA. One signal appeared, representing a DNA fragment of 16.5 kb.

[0027] A λL47 gene bank from C. vinosum genomic DNA was prepared and plates with approximately 800 plaques were blotted on to nylon membranes and hybridized with biotinylated SE52 DNA. One positive recombinant phage was isolated, which harboured a 16.5 kb EcoRl fragment, which was designated PP10 (FIG. 1). With PP10 and a 9.4 kb EcoRI/PstI subfragment (EP94) of PP10, the phenotype of the wild type could be restored in PHA-negative mutants of A. eutrophus.

[0028] Expression studies in E. coli (see below) showed that a 4.5 kb SmaI/EcoRI (SE45) subfragment of EP94 encodes for PHA synthase and β-ketothiolase. The nucleotide sequence of this fragment was determined by-the dideoxy-chain termination method of Sanger et al. with alkaline denatured double stranded plasmid DNA. The T7-polymerase sequencing kit of Pharmacia, Uppsala, Sweden, was used with 7-deazaguanosine-5′-triphosphate instead of dGTP. Most of the sequence was determined with a set of unidirectional overlapping deletion clones generated by exonuclease III digestion. For sequencing regions which were not covered by the deletion plasmids synthetic oligonucleotides were used.

[0029] It was not possible to clone the 4.9 kb SmaI/PstI fragment PS49 in a multi-copy vector. Therefore, fragment EP94 (FIG. 1) was treated with Exonuclease Bal31, ligated to Bluescript SK and transferred to E. coli xl-1 Blue. A clone was isolated which harboured Bluescript SK with a 5.5 kb fragment (B55) and which expressed β-ketothiolase and NADH-dependent reductase activity. 3146 base pairs of B55 were part of the SE45 fragment. The other part (approximately 2350 base pairs, SB24) has been sequenced applying the primer hopping strategy. The sequence and the position of the reductase gene on SB24 are known. The results of these studies, including the organisation of the PHA biosynthetic genes in C. vinosum and the sites of the ketothiolase, reductase and PHA synthase genes are shown in FIG. 2. The determination of the full sequence of SB24 is in progress.

2. Sequence Analysis of the C. vinosum PHB Synthetic Genes

[0030] The nucleotide sequence of SE45 is shown in FIG. 3. The fragment size of SE45 is 4506 bp.

2.1 PHB Synthase

[0031] The fragment sequence corresponding to the PHB synthase gene is designated as ORF3. The determination of synthase activity of deleted plasmids containing SE45 (See below) gave evidence that expression of ORF2 is also required for expression of synthase activity.

[0032] ORF2 and ORF3 are transcribed as an operon. The determination of the transcription start site of ORF2 was conducted by S1 nuclease mapping. This site occurs at bp 3059 from the 3′ end of SE45. A putative “−10” site, given as 5′-ACAGAT-3 ′ occurs at bp 3073-3078, and a “−35” site occurs at bp 3092-3099. A putative ribosome binding site occurs at bp 3040-3045. The translation start codon commences at bp 3030. The translation stop codon occurs at bp 1958.

[0033] The putative ribosome binding site of ORF3 occurs at bp 1907-1911. The translation start ATG for ORF3 occurs at bp 1899, and the translation stop codon at bp 833. Putative transcriptional terminator sites occur at hairpin structures at bp 773-786 and 796-823.

[0034] ORF2 encodes a polypeptide of 358 amino acids with a MW of 40525 da. ORF3 encodes a polypeptide of 356 amino acids with a MW of 39739 da. The gene size of ORF3 is approximately 30% smaller as compared with the PHA polymerase genes of A. eutrophus and P. oleovorans. The alignments of the primary structures of C. vinosum PHA polymerase, A. eutrophus PHA polymerase and P. oleovorans PHA polymerases 1 and 2 are shown in FIG. 4. Thus the ORF3 C. vinosum polymerase is shorter than the other polymerase enzymes, lacking the first 172 amino acids from the NH2 terminus of the A. eutrophus PHA polymerase, and the first 148 amino acids of the Pseudomonas polymerases. The amino acid sequence of ORF3 exhibited an overall homology of 25% to the polymerase of A. eutrophus, with certain discrete regions of conserved sequence.

[0035] The amino acid sequence of ORF2showed no significant homology to other enzymes in the NBRF gene bank.

2.2 β ketothiolase

[0036] The β ketothiolase and acetoacetyl CoA reductase genes are transcribed in opposite direction to ORF2 and ORF3 (FIG. 2). A “−10” site in the identified ketothiolase promoter occurs at bp 3105-3111, and a “−35” site at bp 3082-3086. A putative ribosome binding site occurs at bp 3167-3171. The translation starts signal occurs at bp 3181. The translation stop codon occurs at bp 4361.

[0037] The aligments of the primary structures of β ketothiolases from Chromatium vinosum and other sources are shown in FIG. 5. Considerable homology is apparent betwen the amino acid sequences of ketothiolases from C. vinosum and other bacterial and non-bacterial sources.

2.3 Acetoacetyl CoA Reductase

[0038] The alignments of the primary structures of acetoacetyl CoA reductases from C. vinosum, A. eutrophus and Z. ramigera are shown in FIG. 6. All three reductases are of similar chain length, while considerable homology is apparent between the sequences of reductases from these bacteria.

[0039] The Chromatium vinosum PHA synthetic genes therefore differ from the PHA synthetic genes of A. eutrophus and P. oleovorans in the following respects:

[0040] i) Whereas A. eutrophus PHB polymerase, acetoacetyl CoA reductase and β ketothiolase genes are all transcribed as an operon, in C. vinosum the ketothiolase and reductase genes are transcribed separately from the polymerase, and are transcribed in the opposite direction to the polymerase ORF3 and ORF2 genes.

[0041] ii) In contrast to A. eutrophus, where one gene product is required for polymerase activity, in C. vinosum two gene products, represented by ORF2 and ORF3 are required for expression of polymerase activity.

[0042] iii) The C. vinosum ORF3 polymerase is 172 amino acids shorter, at the amino terminus, than the A. eutrophus polymerase, and 148 amino acids shorter than the P. oleovorans polymerases 1 and 2. The C. vinosum ORF3 shows only 25% homology with the primary sequence of the A. eutrophus polymerase.

[0043] iv) The A. eutrophus acetoacetyl CoA reductase enzyme involved in PHB synthesis is NADPH specific, while the C. vinosum enzyme exhibits a marked preference for NADH.

[0044] Between the structural genes for ketothiolase and acetoacetyl CoA reductase of Chromatium vinosum, two open reading frames (ORF4 and ORF5) appear, and downstream from the reductase gene an ORF7 has been identified (FIG. 5). No additional ORFs were identified in the PHA coding region of A. eutrophus.

3. Expression of C. vinosum PHB Synthetic Genes in Other Bacteria.

[0045] With fragments PP10 and EP94 the ability to synthesise PHB could be restored to PHB negative mutants of A. eutrophus. Recombinant strains of the PHB negative mutant A. eutrophus PHB-4, transformed with these fragments, were able to synthesise polymers containing 3-hydroxybutyrate and 3-hydroxyisovalerate at significant proportions, when supplied with appropriate substrates.

[0046] Studies on expression of C. vinosum DNA fragments in E. coli are presented in Table 1. Thus E. coli transformed with plasmids containing fragments PP10 and EP94 expressed PHB polymerase, acetoacetyl CoA reductase and β ketothiolase activities. They also synthesised PHB up to between 10 and 12% dry weight. E. coli transformed with plasmids containing fragment SE45 expressed PHB polymerase and β ketothiolase, but not acetoacetyl CoA reductase, and were unable to synthesise PHB.

4. Polymer Biochemistry

[0047] The specific optical rotations of methyl 3-hydroxybutyric acid liberated by methanolysis of PHB from C. vinosum (accumulated from acetate), from A. eutrophus PHB-4 pHP1014::PP10 (accumulated from fructose) and E. coli S17-1 pSUP202::PP10 (accumulated from glucose) were all negative. The determined values of the specific optical rotation were similar to those for PHB isolated from A. eutrophus (accumulated from fructose).

Claims

1. Gene fragments isolated from the bacterium Chromatium vinosum and encoding polyhydroxyalkanoate (PHA) synthase, acetoacetyl CoA reductase and β-ketothiolase.

2. Gene fragments as claimed in claim 1 in which the Chromatium vinosum is of the strain designated D, available to the public from the Deutsche Sammlung fur Mikroorganismen under the Accession Number 180.

3. A 16.5kb EcoRl fragment of Chromatium vinosum DNA, designated PP10, hybridizable to a 5.2 kb SmaI/EcoRl fragment, designated SE52 isolated from Alcaligenes eutrophus and known to contain the genes encoding PHA-synthase acetoacetyl CoA reductase and β-ketothiolase.

4. A fragment of the PP10 fragment claimed in claim 3, designated SE45, encoding the PHA-synthase and β-ketothiolase genes.

5. A fragment of the PP10 fragment claimed in claim 3, designated SB24, encoding the acetoacetyl CoA reductase gene.

6. A recombinant genome which contains one or more of the fragments designated PP10, SE45 and region SB24 claimed in claims 3, 4 and 5 respectively.

7. A bacterium having incorporated in its genome one or more of the fragments designated PP10, SE45 and region SB24 claimed in claims 3, 4 and 5 respectively.

8. A plant having stably incorporated in its genome by transformation one or more of the fragments designated PP10, SE45 and region SB24 claimed in claims 3, 4 and 5 respectively.

9. A method for the manufacture of polyhydroxyalkanoates, comprising culturing the microorganism Chromatium vinosum, or a bacterium of a different species having stably incorporated within its genome by transformation one or more PHA synthesising genes from Chromatium vinosum.

10. A gene, encoding β-ketothiolase, having the nucleotide sequence shown in FIG. 3.

11. A gene encoding polyhydroxyalkanoate synthase (phbC), having the nucleotide sequence shown in FIG. 5.

12. A gene encoding acetoacetyl CoA reductase (phbB) having the nucleotide sequence shown in FIG. 5.