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1.  Rearrangement of Gene Order in the phaCAB Operon Leads to Effective Production of Ultrahigh-Molecular-Weight Poly[(R)-3-Hydroxybutyrate] in Genetically Engineered Escherichia coli 
Ultrahigh-molecular-weight poly[(R)-3-hydroxybutyrate] [UHMW-P(3HB)] synthesized by genetically engineered Escherichia coli is an environmentally friendly bioplastic material which can be processed into strong films or fibers. An operon of three genes (organized as phaCAB) encodes the essential proteins for the production of P(3HB) in the native producer, Ralstonia eutropha. The three genes of the phaCAB operon are phaC, which encodes the polyhydroxyalkanoate (PHA) synthase, phaA, which encodes a 3-ketothiolase, and phaB, which encodes an acetoacetyl coenzyme A (acetoacetyl-CoA) reductase. In this study, the effect of gene order of the phaCAB operon (phaABC, phaACB, phaBAC, phaBCA, phaCAB, and phaCBA) on an expression plasmid in genetically engineered E. coli was examined in order to determine the best organization to produce UHMW-P(3HB). The results showed that P(3HB) molecular weights and accumulation levels were both dependent on the order of the pha genes relative to the promoter. The most balanced production result was achieved in the strain harboring the phaBCA expression plasmid. In addition, analysis of expression levels and activity for P(3HB) biosynthesis enzymes and of P(3HB) molecular weight revealed that the concentration of active PHA synthase had a negative correlation with P(3HB) molecular weight and a positive correlation with cellular P(3HB) content. This result suggests that the level of P(3HB) synthase activity is a limiting factor for producing UHMW-P(3HB) and has a significant impact on P(3HB) production.
PMCID: PMC3346486  PMID: 22344649
2.  Connection between Poly-β-Hydroxybutyrate Biosynthesis and Growth on C1 and C2 Compounds in the Methylotroph Methylobacterium extorquens AM1 
Journal of Bacteriology  2001;183(3):1038-1046.
Several DNA regions containing genes involved in poly-β-hydroxybutyrate (PHB) biosynthesis and degradation and also in fatty acid degradation were identified from genomic sequence data and have been characterized in the serine cycle facultative methylotroph Methylobacterium extorquens AM1. Genes involved in PHB biosynthesis include those encoding β-ketothiolase (phaA), NADPH-linked acetoacetyl coenzyme A (acetyl-CoA) reductase (phaB), and PHB synthase (phaC). phaA and phaB are closely linked on the chromosome together with a third gene with identity to a regulator of PHB granule-associated protein, referred to as orf3. phaC was unlinked to phaA and phaB. Genes involved in PHB degradation include two unlinked genes predicted to encode intracellular PHB depolymerases (depA and depB). These genes show a high level of identity with each other at both DNA and amino acid levels. In addition, a gene encoding β-hydroxybutyrate dehydrogenase (hbd) was identified. Insertion mutations were introduced into depA, depB, phaA, phaB, phaC, and hbd and also in a gene predicted to encode crotonase (croA), which is involved in fatty acid degradation, to investigate their role in PHB cycling. Mutants in depA, depB, hbd, and croA all produced normal levels of PHB, and the only growth phenotype observed was the inability of the hbd mutant to grow on β-hydroxybutyrate. However, the phaA, phaB, and phaC mutants all showed defects in PHB synthesis. Surprisingly, these mutants also showed defects in growth on C1 and C2 compounds and, for phaB, these defects were rescued by glyoxylate supplementation. These results suggest that β-hydroxybutyryl-CoA is an intermediate in the unknown pathway that converts acetyl-CoA to glyoxylate in methylotrophs and Streptomyces spp.
PMCID: PMC94972  PMID: 11208803
3.  Roles of Multiple Acetoacetyl Coenzyme A Reductases in Polyhydroxybutyrate Biosynthesis in Ralstonia eutropha H16 ▿ †  
Journal of Bacteriology  2010;192(20):5319-5328.
The bacterium Ralstonia eutropha H16 synthesizes polyhydroxybutyrate (PHB) from acetyl coenzyme A (acetyl-CoA) through reactions catalyzed by a β-ketothiolase (PhaA), an acetoacetyl-CoA reductase (PhaB), and a polyhydroxyalkanoate synthase (PhaC). An operon of three genes encoding these enzymatic steps was discovered in R. eutropha and has been well studied. Sequencing and analysis of the R. eutropha genome revealed putative isologs for each of the PHB biosynthetic genes, many of which had never been characterized. In addition to the previously identified phaB1 gene, the genome contains the isologs phaB2 and phaB3 as well as 15 other potential acetoacetyl-CoA reductases. We have investigated the roles of the three phaB isologs by deleting them from the genome individually and in combination. It was discovered that the gene products of both phaB1 and phaB3 contribute to PHB biosynthesis in fructose minimal medium but that in plant oil minimal medium and rich medium, phaB3 seems to be unexpressed. This raises interesting questions concerning the regulation of phaB3 expression. Deletion of the gene phaB2 did not result in an observable phenotype under the conditions tested, although this gene does encode an active reductase. Addition of the individual reductase genes to the genome of the ΔphaB1 ΔphaB2 ΔphaB3 strain restored PHB production, and in the course of our complementation experiments, we serendipitously created a PHB-hyperproducing mutant. Measurement of the PhaB and PhaA activities of the mutant strains indicated that the thiolase reaction is the limiting step in PHB biosynthesis in R. eutropha H16 during nitrogen-limited growth on fructose.
PMCID: PMC2950492  PMID: 20729355
4.  Polyhydroxyalkanoate production in Rhodobacter capsulatus: genes, mutants, expression, and physiology. 
Like many other prokaryotes, the photosynthetic bacterium Rhodobacter capsulatus produces high levels of polyhydroxyalkanoates (PHAs) when a suitable carbon source is available. The three genes that are traditionally considered to be necessary in the PHA biosynthetic pathway, phaA (beta-ketothiolase), phaB (acetoacetylcoenzyme A reductase), and phaC (PHA synthase), were cloned from Rhodobacter capsulatus. In R. capsulatus, the phaAB genes are not linked to the phaC gene. Translational beta-galactosidase fusions to phaA and phaC were constructed and recombined into the chromosome. Both phaC and phaA were constitutively expressed regardless of whether PHA production was induced, suggesting that control is posttranslational at the enzymatic level. Consistent with this conclusion, it was shown that the R. capsulatus transcriptional nitrogen-sensing circuits were not involved in PHA synthesis. The doubling times of R. capsulatus transcriptional nitrogen-sensing circuits were not involved in PHA synthesis. The doubling times of R. capsulatus grown on numerous carbon sources were determined, indicating that this bacterium grows on C2 to C12 fatty acids. Grown on acetone, caproate, or heptanoate, wild-type R. capsulatus produced high levels of PHAs. Although a phaC deletion strain was unable to synthesize PHAs on any carbon source, phaA and phaAB deletion strains were able to produce PHAs, indicating that alternative routes for the synthesis of substrates for the synthase are present. The nutritional versatility and bioenergetic versatility of R. capsulatus, coupled with its ability to produce large amounts of PHAs and its genetic tractability, make it an attractive model for the study of PHA production.
PMCID: PMC168600  PMID: 9251189
5.  Characterization of the Highly Active Polyhydroxyalkanoate Synthase of Chromobacterium sp. Strain USM2▿ 
The synthesis of bacterial polyhydroxyalkanoates (PHA) is very much dependent on the expression and activity of a key enzyme, PHA synthase (PhaC). Many efforts are being pursued to enhance the activity and broaden the substrate specificity of PhaC. Here, we report the identification of a highly active wild-type PhaC belonging to the recently isolated Chromobacterium sp. USM2 (PhaCCs). PhaCCs showed the ability to utilize 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3HV), and 3-hydroxyhexanoate (3HHx) monomers in PHA biosynthesis. An in vitro assay of recombinant PhaCCs expressed in Escherichia coli showed that its polymerization of 3-hydroxybutyryl-coenzyme A activity was nearly 8-fold higher (2,462 ± 80 U/g) than that of the synthase from the model strain C. necator (307 ± 24 U/g). Specific activity using a Strep2-tagged, purified PhaCCs was 238 ± 98 U/mg, almost 5-fold higher than findings of previous studies using purified PhaC from C. necator. Efficient poly(3-hydroxybutyrate) [P(3HB)] accumulation in Escherichia coli expressing PhaCCs of up to 76 ± 2 weight percent was observed within 24 h of cultivation. To date, this is the highest activity reported for a purified PHA synthase. PhaCCs is a naturally occurring, highly active PHA synthase with superior polymerizing ability.
PMCID: PMC3126384  PMID: 21398494
6.  Wide Distribution among Halophilic Archaea of a Novel Polyhydroxyalkanoate Synthase Subtype with Homology to Bacterial Type III Synthases▿ †  
Applied and Environmental Microbiology  2010;76(23):7811-7819.
Polyhydroxyalkanoates (PHAs) are accumulated as intracellular carbon and energy storage polymers by various bacteria and a few haloarchaea. In this study, 28 strains belonging to 15 genera in the family Halobacteriaceae were investigated with respect to their ability to synthesize PHAs and the types of their PHA synthases. Fermentation results showed that 18 strains from 12 genera could synthesize polyhydroxybutyrate (PHB) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). For most of these haloarchaea, selected regions of the phaE and phaC genes encoding PHA synthases (type III) were cloned via PCR with consensus-degenerate hybrid oligonucleotide primers (CODEHOPs) and were sequenced. The PHA synthases were also examined by Western blotting using haloarchaeal Haloarcula marismortui PhaC (PhaCHm) antisera. Phylogenetic analysis showed that the type III PHA synthases from species of the Halobacteriaceae and the Bacteria domain clustered separately. Comparison of their amino acid sequences revealed that haloarchaeal PHA synthases differed greatly in both molecular weight and certain conserved motifs. The longer C terminus of haloarchaeal PhaC was found to be indispensable for its enzymatic activity, and two additional amino acid residues (C143 and C190) of PhaCHm were proved to be important for its in vivo function. Thus, we conclude that a novel subtype (IIIA) of type III PHA synthase with unique features that distinguish it from the bacterial subtype (IIIB) is widely distributed in haloarchaea and appears to be involved in PHA biosynthesis.
PMCID: PMC2988587  PMID: 20889776
7.  Directed Evolution and Structural Analysis of NADPH-Dependent Acetoacetyl Coenzyme A (Acetoacetyl-CoA) Reductase from Ralstonia eutropha Reveals Two Mutations Responsible for Enhanced Kinetics 
Applied and Environmental Microbiology  2013;79(19):6134-6139.
NADPH-dependent acetoacetyl-coenzyme A (acetoacetyl-CoA) reductase (PhaB) is a key enzyme in the synthesis of poly(3-hydroxybutyrate) [P(3HB)], along with β-ketothiolase (PhaA) and polyhydroxyalkanoate synthase (PhaC). In this study, PhaB from Ralstonia eutropha was engineered by means of directed evolution consisting of an error-prone PCR-mediated mutagenesis and a P(3HB) accumulation-based in vivo screening system using Escherichia coli. From approximately 20,000 mutants, we obtained two mutant candidates bearing Gln47Leu (Q47L) and Thr173Ser (T173S) substitutions. The mutants exhibited kcat values that were 2.4-fold and 3.5-fold higher than that of the wild-type enzyme, respectively. In fact, the PhaB mutants did exhibit enhanced activity and P(3HB) accumulation when expressed in recombinant Corynebacterium glutamicum. Comparative three-dimensional structural analysis of wild-type PhaB and highly active PhaB mutants revealed that the beneficial mutations affected the flexibility around the active site, which in turn played an important role in substrate recognition. Furthermore, both the kinetic analysis and crystal structure data supported the conclusion that PhaB forms a ternary complex with NADPH and acetoacetyl-CoA. These results suggest that the mutations affected the interaction with substrates, resulting in the acquirement of enhanced activity.
PMCID: PMC3811355  PMID: 23913421
8.  Mutations Derived from the Thermophilic Polyhydroxyalkanoate Synthase PhaC Enhance the Thermostability and Activity of PhaC from Cupriavidus necator H16 
Journal of Bacteriology  2012;194(10):2620-2629.
The thermophile Cupriavidus sp. strain S-6 accumulated polyhydroxybutyrate (PHB) from glucose at 50°C. A 9.0-kbp EcoRI fragment cloned from the genomic DNA of Cupriavidus sp. S-6 enabled Escherichia coli XL1-Blue to synthesize PHB at 45°C. Nucleotide sequence analysis showed a pha locus in the clone. The thermophilic polyhydroxyalkanoate (PHA) synthase (PhaCCsp) shared 81% identity with mesophilic PhaC of Cupriavidus necator H16. The diversity between these two strains was found dominantly on their N and C termini, while the middle regions were highly homologous (92% identity). We constructed four chimeras of mesophilic and thermophilic phaC genes to explore the mutations related to its thermostability. Among the chimeras, only PhaCH16β, which was PhaCH16 bearing 30 point mutations derived from the middle region of PhaCCsp, accumulated a high content of PHB (65% [dry weight]) at 45°C. The chimera phaCH16β and two parental PHA synthase genes were overexpressed in E. coli BLR(DE3) cells and purified. At 30°C, the specific activity of the chimera PhaCH16β (172 ± 17.8 U/mg) was 3.45-fold higher than that of the parental enzyme PhaCH16 (50 ± 5.2 U/mg). At 45°C, the half-life of the chimera PhaCH16β (11.2 h) was 127-fold longer than that of PhaCH16 (5.3 min). Furthermore, the chimera PhaCH16β accumulated 1.55-fold (59% [dry weight]) more PHA content than the parental enzyme PhaCH16 (38% [dry weight]) at 37°C. This study reveals a limited number of point mutations which enhance not only thermostability but also PhaCH16 activity. The highly thermostable and active PHA synthase will provide advantages for its promising applications to in vitro PHA synthesis and recombinant E. coli PHA fermentation.
PMCID: PMC3347165  PMID: 22408158
9.  The Ralstonia eutropha PhaR Protein Couples Synthesis of the PhaP Phasin to the Presence of Polyhydroxybutyrate in Cells and Promotes Polyhydroxybutyrate Production 
Journal of Bacteriology  2002;184(1):59-66.
Polyhydroxyalkanoates (PHAs) are polyoxoesters that are produced by many bacteria and that accumulate as intracellular granules. Phasins (PhaP) are proteins that accumulate during PHA synthesis, bind PHA granules, and promote further PHA synthesis. Interestingly, PhaP accumulation seems to be strictly dependent on PHA synthesis, which is catalyzed by the PhaC PHA synthase. Here we have tested the effect of the Ralstonia eutropha PhaR protein on the regulation of PhaP accumulation. R. eutropha strains with phaR, phaC, and/or phaP deletions were constructed, and PhaP accumulation was measured by immunoblotting. The wild-type strain accumulated PhaP in a manner dependent on PHA production, and the phaC deletion strain accumulated no PhaP, as expected. In contrast, both the phaR and the phaR phaC deletion strains accumulated PhaP to higher levels than did the wild type. This result implies that PhaR is a negative regulator of PhaP accumulation and that PhaR specifically prevents PhaP from accumulating in cells that are not producing PHA. Transfer of the R. eutropha phaR, phaP, and PHA biosynthesis (phaCAB) genes into a heterologous system, Escherichia coli, was sufficient to reconstitute the PhaR/PhaP regulatory system, implying that PhaR both regulates PhaP accumulation and responds to PHA directly. Deletion of phaR caused a decrease in PHA yields, and a phaR phaP deletion strain exhibited a more severe PHA defect than a phaP deletion strain, implying that PhaR promotes PHA production and does this at least partially through a PhaP-independent pathway. Models for regulatory roles of PhaR in regulating PhaP and promoting PHA production are presented.
PMCID: PMC134771  PMID: 11741844
10.  Comparison of four phaC genes from Haloferax mediterranei and their function in different PHBV copolymer biosyntheses in Haloarcula hispanica 
Saline Systems  2010;6:9.
The halophilic archaeon Haloferax mediterranei is able to accumulate large amounts of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with high molar fraction of 3-hydroxyvalerate (3HV) from unrelated carbon sources. A Polyhydroxyalkanoate (PHA) synthase composed of two subunits, PhaCHme and PhaEHme, has been identified in this strain, and shown to account for the PHBV biosynthesis.
With the aid of the genome sequence of Hfx. mediterranei CGMCC 1.2087, three additional phaC genes (designated phaC1, phaC2, and phaC3) were identified, which encoded putative PhaCs. Like PhaCHme (54.8 kDa), PhaC1 (49.7 kDa) and PhaC3 (62.5 kDa) possessed the conserved motifs of type III PHA synthase, which was not observed in PhaC2 (40.4 kDa). Furthermore, the longer C terminus found in the other three PhaCs was also absent in PhaC2. Reverse transcription PCR (RT-PCR) revealed that, among the four genes, only phaCHme was transcribed under PHA-accumulating conditions in the wild-type strain. However, heterologous coexpression of phaEHme with each phaC gene in Haloarcula hispanica PHB-1 showed that all PhaCs, except PhaC2, could lead to PHBV accumulation with various 3HV fractions. The three kinds of copolymers were characterized using gel-permeation chromatography (GPC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Their thermal properties changed with the variations in monomer composition as well as the different molecular weights (Mw), thus might meet various application requirements.
We discover three cryptic phaC genes in Hfx. mediterranei, and demonstrate that genetic engineering of these newly identified phaC genes has biotechnological potential for PHBV production with tailor-made material properties.
PMCID: PMC2939530  PMID: 20727166
11.  Evaluating PHA Productivity of Bioengineered Rhodosprillum rubrum 
PLoS ONE  2014;9(5):e96621.
This study explored the potential of using Rhodosprillum rubrum as the biological vehicle to convert chemically simple carbon precursors to a value-added bio-based product, the biopolymer PHA. R. rubrum strains were bioengineered to overexpress individually or in various combinations, six PHA biosynthetic genes (phaC1, phaA, phaB, phaC2, phaC3, and phaJ), and the resulting nine over-expressing strains were evaluated to assess the effect on PHA content, and the effect on growth. These experiments were designed to genetically evaluate: 1) the role of each apparently redundant PHA polymerase in determining PHA productivity; 2) identify the key gene(s) within the pha biosynthetic operon that determines PHA productivity; and 3) the role of phaJ to support PHA productivity. The result of overexpressing each PHA polymerase-encoding gene indicates that phaC1 and phaC2 are significant contributors to PHA productivity, whereas phaC3 has little effect. Similarly, over-expressing individually or in combination the three PHA biosynthesis genes located in the pha operon indicates that phaB is the key determinant of PHA productivity. Finally, analogous experiments indicate that phaJ does not contribute significantly to PHA productivity. These bioengineering strains achieved PHA productivity of up to 30% of dry biomass, which is approximately 2.5-fold higher than the non-engineered control strain, indicating the feasibility of using this approach to produce value added bio-based products.
PMCID: PMC4026134  PMID: 24840941
12.  Characterization of Site-Specific Mutations in a Short-Chain-Length/Medium-Chain-Length Polyhydroxyalkanoate Synthase: In Vivo and In Vitro Studies of Enzymatic Activity and Substrate Specificity 
Applied and Environmental Microbiology  2013;79(12):3813-3821.
Saturation point mutagenesis was carried out at position 479 in the polyhydroxyalkanoate (PHA) synthase from Chromobacterium sp. strain USM2 (PhaCCs) with specificities for short-chain-length (SCL) [(R)-3-hydroxybutyrate (3HB) and (R)-3-hydroxyvalerate (3HV)] and medium-chain-length (MCL) [(R)-3-hydroxyhexanoate (3HHx)] monomers in an effort to enhance the specificity of the enzyme for 3HHx. A maximum 4-fold increase in 3HHx incorporation and a 1.6-fold increase in PHA biosynthesis, more than the wild-type synthase, was achieved using selected mutant synthases. These increases were subsequently correlated with improved synthase activity and increased preference of PhaCCs for 3HHx monomers. We found that substitutions with uncharged residues were beneficial, as they resulted in enhanced PHA production and/or 3HHx incorporation. Further analysis led to postulations that the size and geometry of the substrate-binding pocket are determinants of PHA accumulation, 3HHx fraction, and chain length specificity. In vitro activities for polymerization of 3HV and 3HHx monomers were consistent with in vivo substrate specificities. Ultimately, the preference shown by wild-type and mutant synthases for either SCL (C4 and C5) or MCL (C6) substrates substantiates the fundamental classification of PHA synthases.
PMCID: PMC3675958  PMID: 23584780
13.  Accumulation of the PhaP Phasin of Ralstonia eutropha Is Dependent on Production of Polyhydroxybutyrate in Cells 
Journal of Bacteriology  2001;183(14):4217-4226.
Polyhydroxyalkanoates (PHAs) are polyoxoesters that are produced by diverse bacteria and that accumulate as intracellular granules. Phasins are granule-associated proteins that accumulate to high levels in strains that are producing PHAs. The accumulation of phasins has been proposed to be dependent on PHA production, a model which is now rigorously tested for the phasin PhaP of Ralstonia eutropha. R. eutropha phaC PHA synthase and phaP phasin gene replacement strains were constructed. The strains were engineered to express heterologous and/or mutant PHA synthase alleles and a phaP-gfp translational fusion in place of the wild-type alleles of phaC and phaP. The strains were analyzed with respect to production of polyhydroxybutyrate (PHB), accumulation of PhaP, and expression of the phaP-gfp fusion. The results suggest that accumulation of PhaP is strictly dependent on the genetic capacity of strains to produce PHB, that PhaP accumulation is regulated at the level of both PhaP synthesis and PhaP degradation, and that, within mixed populations of cells, PhaP accumulation within cells of a given strain is not influenced by PHB production in cells of other strains. Interestingly, either the synthesis of PHB or the presence of relatively large amounts of PHB in cells (>50% of cell dry weight) is sufficient to enable PhaP synthesis. The results suggest that R. eutropha has evolved a regulatory mechanism that can detect the synthesis and presence of PHB in cells and that PhaP expression can be used as a marker for the production of PHB in individual cells.
PMCID: PMC95311  PMID: 11418562
14.  Cloning, Molecular Analysis, and Expression of the Polyhydroxyalkanoic Acid Synthase (phaC) Gene from Chromobacterium violaceum 
The polyhydroxyalkanoic acid synthase gene from Chromobacterium violaceum (phaCCv) was cloned and characterized. A 6.3-kb BamHI fragment was found to contain both phaCCv and the polyhydroxyalkanoic acid (PHA)-specific 3-ketothiolase (phaACv). Escherichia coli strains harboring this fragment produced significant levels of PHA synthase and 3-ketothiolase, as judged by their activities. While C. violaceum accumulated poly(3-hydroxybutyrate) or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) when grown on a fatty acid carbon source, Klebsiella aerogenes and Ralstonia eutropha (formerly Alcaligenes eutrophus), harboring phaCCv, accumulated the above-mentioned polymers and, additionally, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) when even-chain-length fatty acids were utilized as the carbon source. This finding suggests that the metabolic environments of these organisms are sufficiently different to alter the product range of the C. violaceum PHA synthase. Neither recombinant E. coli nor recombinant Pseudomonas putida harboring phaCCv accumulated significant levels of PHA. Sequence analysis of the phaCCv product shows homology with several PHA synthases, most notably a 48% identity with that of Alcaligenes latus (GenBank accession no. AAD10274).
PMCID: PMC91534  PMID: 10427049
15.  Phosphate concentration regulates transcription of the Acinetobacter polyhydroxyalkanoic acid biosynthetic genes. 
Journal of Bacteriology  1995;177(15):4501-4507.
The polyhydroxyalkanoic acid (PHA) biosynthetic gene locus was cloned and characterized from an Acinetobacter sp. isolated from activated sludge. Nucleotide sequence analysis identified three clustered genes, phaAAc (encoding a beta-ketothiolase), phaBAc (encoding an acetoacetyl coenzyme A reductase), and phaCAc (encoding a PHA synthase). In addition, an open reading frame (ORF1) with potential to encode a 13-kDa protein was identified within this locus. The sequence of the putative translational product of ORF1 does not show significant similarity to any sequences in the database. A plasmid containing the Acinetobacter pha locus conferred the ability to accumulate poly-beta-hydroxybutyrate on its Escherichia coli host. These genes appear to lie in an operon transcribed by two promoters upstream of phaBAc, an apparent constitutive promoter, and a second promoter induced by phosphate starvation and under pho regulon control. These as well as a number of additional potential transcription start points were identified by a combination of primer extension and promoter-chloramphenicol acetyltransferase gene fusion studies carried out in Acinetobacter or E. coli transformants.
PMCID: PMC177202  PMID: 7635832
16.  Production of copolyesters of 3-hydroxybutyrate and medium-chain-length 3-hydroxyalkanoates by E. coli containing an optimized PHA synthase gene 
Microbial polyhydroxyalkanoates (PHA) are biopolyesters consisting of diverse monomers. PHA synthase PhaC2Ps cloned from Pseudomonas stutzeri 1317 is able to polymerize short-chain-length (scl) 3-hydroxybutyrate (3HB) monomers and medium-chain-length (mcl) 3-hydroxyalkanoates (3HA) with carbon chain lengths ranging from C6 to C12. However, the scl and mcl PHA production in Escherichia coli expressing PhaC2Ps is limited with very low PHA yield.
To improve the production of PHA with a wide range of monomer compositions in E. coli, a series of optimization strategies were applied on the PHA synthase PhaC2Ps. Codon optimization of the gene and mRNA stabilization with a hairpin structure were conducted and the function of the optimized PHA synthase was tested in E. coli. The transcript was more stable after the hairpin structure was introduced, and western blot analysis showed that both codon optimization and hairpin introduction increased the protein expression level. Compared with the wild type PhaC2Ps, the optimized PhaC2Ps increased poly-3-hydroxybutyrate (PHB) production by approximately 16-fold to 30% of the cell dry weight. When grown on dodecanoate, the recombinant E. coli harboring the optimized gene phaC2PsO with a hairpin structure in the 5’ untranslated region was able to synthesize 4-fold more PHA consisting of 3HB and medium-chain-length 3HA compared to the recombinant harboring the wild type phaC2Ps.
The levels of both PHB and scl-mcl PHA in E. coli were significantly increased by series of optimization strategies applied on PHA synthase PhaC2Ps. These results indicate that strategies including codon optimization and mRNA stabilization are useful for heterologous PHA synthase expression and therefore enhance PHA production.
PMCID: PMC3503839  PMID: 22978778
PHB; Polyhydroxyalkanoates; PHA synthase; Codon optimization; Hairpin; Escherichia coli
17.  PhaC and PhaR Are Required for Polyhydroxyalkanoic Acid Synthase Activity in Bacillus megaterium 
Journal of Bacteriology  2001;183(14):4235-4243.
Polyhydroxyalkanoic acids (PHAs) are a class of polyesters stored in inclusion bodies and found in many bacteria and in some archaea. The terminal step in the synthesis of PHA is catalyzed by PHA synthase. Genes encoding this enzyme have been cloned, and the primary sequence of the protein, PhaC, is deduced from the nucleotide sequences of more than 30 organisms. PHA synthases are grouped into three classes based on substrate range, molecular mass, and whether or not there is a requirement for phaE in addition to the phaC gene product. Here we report the results of an analysis of a PHA synthase that does not fit any of the described classes. This novel PHA synthase from Bacillus megaterium required PhaC (PhaCBm) and PhaR (PhaRBm) for activity in vivo and in vitro. PhaCBm showed greatest similarity to the PhaCs of class III in both size and sequence. Unlike those in class III, the 40-kDa PhaE was not required, and furthermore, the 22-kDa PhaRBm had no obvious homology to PhaE. Previously we showed that PhaCBm, and here we show that PhaRBm, is localized to inclusion bodies in living cells. We show that two forms of PHA synthase exist, an active form in PHA-accumulating cells and an inactive form in nonaccumulating cells. PhaC was constitutively produced in both cell types but was more susceptible to protease degradation in the latter type. Our data show that the role of PhaR is posttranscriptional and that it functions directly or indirectly with PhaCBm to produce an active PHA synthase.
PMCID: PMC95313  PMID: 11418564
18.  Cloning of the Alcaligenes latus Polyhydroxyalkanoate Biosynthesis Genes and Use of These Genes for Enhanced Production of Poly(3-hydroxybutyrate) in Escherichia coli 
Applied and Environmental Microbiology  1998;64(12):4897-4903.
Polyhydroxyalkanoates (PHAs) are microbial polyesters that can be used as completely biodegradable polymers, but the high production cost prevents their use in a wide range of applications. Recombinant Escherichia coli strains harboring the Ralstonia eutropha PHA biosynthesis genes have been reported to have several advantages as PHA producers compared with wild-type PHA-producing bacteria. However, the PHA productivity (amount of PHA produced per unit volume per unit time) obtained with these recombinant E. coli strains has been lower than that obtained with the wild-type bacterium Alcaligenes latus. To endow the potentially superior PHA biosynthetic machinery to E. coli, we cloned the PHA biosynthesis genes from A. latus. The three PHA biosynthesis genes formed an operon with the order PHA synthase, β-ketothiolase, and reductase genes and were constitutively expressed from the natural promoter in E. coli. Recombinant E. coli strains harboring the A. latus PHA biosynthesis genes accumulated poly(3-hydroxybutyrate) (PHB), a model PHA product, more efficiently than those harboring the R. eutropha genes. With a pH-stat fed-batch culture of recombinant E. coli harboring a stable plasmid containing the A. latus PHA biosynthesis genes, final cell and PHB concentrations of 194.1 and 141.6 g/liter, respectively, were obtained, resulting in a high productivity of 4.63 g of PHB/liter/h. This improvement should allow recombinant E. coli to be used for the production of PHB with a high level of economic competitiveness.
PMCID: PMC90940  PMID: 9835580
19.  Genetic Analysis of Comamonas acidovorans Polyhydroxyalkanoate Synthase and Factors Affecting the Incorporation of 4-Hydroxybutyrate Monomer 
The polyhydroxyalkanoate (PHA) synthase gene of Comamonas acidovorans DS-17 (phaCCa) was cloned by using the synthase gene of Alcaligenes eutrophus as a heterologous hybridization probe. Complete sequencing of a 4.0-kbp SmaI-HindIII (SH40) subfragment revealed the presence of a 1,893-bp PHA synthase coding region which was followed by a 1,182-bp β-ketothiolase gene (phaACa). Both the translated products of these genes showed significant identity, 51.1 and 74.2%, respectively, to the primary structures of the products of the corresponding genes in A. eutrophus. The arrangement of PHA biosynthesis genes in C. acidovorans was also similar to that in A. eutrophus except that the third gene, phaB, coding for acetoacetyl-coenzyme A reductase, was not found in the region downstream of phaACa. The cloned fragment complemented a PHA-negative mutant of A. eutrophus, PHB−4, resulting in poly-3-hydroxybutyrate accumulation of up to 73% of the dry cell weight when fructose was the carbon source. The heterologous expression enabled the incorporation of 4-hydroxybutyrate (4HB) and 3-hydroxyvalerate monomers. The PHA synthase of C. acidovorans does not appear to show any preference for 4-hydroxybutyryl-coenzyme A as a substrate. This leads to the suggestion that in C. acidovorans, it is the metabolic pathway, and not the specificity of the organism’s PHA synthase, that drives the incorporation of 4HB monomers, resulting in the efficient accumulation of PHA with a high 4HB content.
PMCID: PMC106744  PMID: 9726894
20.  Engineering of Chimeric Class II Polyhydroxyalkanoate Synthases 
Applied and Environmental Microbiology  2004;70(11):6789-6799.
PHA synthase is a key enzyme involved in the biosynthesis of polyhydroxyalkanoates (PHAs). Using a combinatorial genetic strategy to create unique chimeric class II PHA synthases, we have obtained a number of novel chimeras which display improved catalytic properties. To engineer the chimeric PHA synthases, we constructed a synthetic phaC gene from Pseudomonas oleovorans (phaC1Po) that was devoid of an internal 540-bp fragment. Randomly amplified PCR products (created with primers based on conserved phaC sequences flanking the deleted internal fragment) were generated using genomic DNA isolated from soil and were substituted for the 540-bp internal region. The chimeric genes were expressed in a PHA-negative strain of Ralstonia eutropha, PHB−4 (DSM 541). Out of 1,478 recombinant clones screened for PHA production, we obtained five different chimeric phaC1Po genes that produced more PHA than the native phaC1Po. Chimeras S1-71, S4-8, S5-58, S3-69, and S3-44 exhibited 1.3-, 1.4-, 2.0-, 2.1-, and 3.0-fold-increased levels of in vivo activity, respectively. All of the mutants mediated the synthesis of PHAs with a slightly increased molar fraction of 3-hydroxyoctanoate; however, the weight-average molecular weights (Mw) of the PHAs in all cases remained almost the same. Based upon DNA sequence analyses, the various phaC fragments appear to have originated from Pseudomonas fluorescens and Pseudomonas aureofaciens. The amino acid sequence analyses showed that the chimeric proteins had 17 to 20 amino acid differences from the wild-type phaC1Po, and these differences were clustered in the same positions in the five chimeric clones. A threading model of PhaC1Po, developed based on homology of the enzyme to the Burkholderia glumae lipase, suggested that the amino acid substitutions found in the active chimeras were located mostly on the protein model surface. Thus, our combinatorial genetic engineering strategy proved to be broadly useful for improving the catalytic activities of PHA synthase enzymes.
PMCID: PMC525123  PMID: 15528546
21.  Cloning and Molecular Analysis of the Poly(3-hydroxybutyrate) and Poly(3-hydroxybutyrate-co-3-hydroxyalkanoate) Biosynthesis Genes in Pseudomonas sp. Strain 61-3 
Journal of Bacteriology  1998;180(24):6459-6467.
Two types of polyhydroxyalkanoate (PHA) biosynthesis gene loci (phb and pha) of Pseudomonas sp. strain 61-3, which produces a blend of poly(3-hydroxybutyrate) [P(3HB)] homopolymer and a random copolymer {poly(3-hydroxybutyrate-co-3-hydroxyalkanoate) [P(3HB-co-3HA]} consisting of 3HA units of 4 to 12 carbon atoms, were cloned and analyzed at the molecular level. In the phb locus, three open reading frames encoding polyhydroxybutyrate (PHB) synthase (PhbCPs), β-ketothiolase (PhbAPs), and NADPH-dependent acetoacetyl coenzyme A reductase (PhbBPs) were found. The genetic organization showed a putative promoter region, followed by phbBPs-phbAPs-phbCPs. Upstream from phbBPs was found the phbRPs gene, which exhibits significant similarity to members of the AraC/XylS family of transcriptional activators. The phbRPs gene was found to be transcribed in the opposite direction from the three structural genes. Cloning of phbRPs in a relatively high-copy vector in Pseudomonas sp. strain 61-3 elevated the levels of β-galactosidase activity from a transcriptional phb promoter-lacZ fusion and also enhanced the 3HB fraction in the polyesters synthesized by this strain, suggesting that PhbRPs is a positive regulatory protein controlling the transcription of phbBACPs in this bacterium. In the pha locus, two genes encoding PHA synthases (PhaC1Ps and PhaC2Ps) were flanked by a PHA depolymerase gene (phaZPs), and two adjacent open reading frames (ORF1 and phaDPs), and the gene order was ORF1, phaC1Ps, phaZPs, phaC2Ps, and phaDPs. Heterologous expression of the cloned fragments in PHA-negative mutants of Pseudomonas putida and Ralstonia eutropha revealed that PHB synthase and two PHA synthases of Pseudomonas sp. strain 61-3 were specific for short chain length and both short and medium chain length 3HA units, respectively.
PMCID: PMC107745  PMID: 9851987
22.  Enhanced Incorporation of 3-Hydroxy-4-Methylvalerate Unit into Biosynthetic Polyhydroxyalkanoate Using Leucine as a Precursor 
AMB Express  2011;1:6.
Ralstonia eutropha PHB-4 expressing Pseudomonas sp. 61-3 polyhydroxyalkanoate (PHA) synthase 1 (PhaC1Ps) synthesizes PHA copolymer containing 3-hydroxybutyrate (3HB) and a small amount (0.5 mol%) of 3-hydroxy-4-methylvalerate (3H4MV) from fructose as a carbon source. In this study, enhanced incorporation of 3H4MV into PHA was investigated using branched amino acid leucine as a precursor of 3H4MV. Leucine has the same carbon backbone as 3H4MV and is expected to be a natural and self-producible precursor. We found that the incorporation of 3H4MV was enhanced by the supplementation of excess amount (10 g/L) of leucine in the culture medium. This finding indicates that 3H4MV can be derived from leucine. To increase metabolic flux to leucine biosynthesis in the host strain by eliminating the feedback inhibition, the cells were subjected to N-methyl-N'-nitro-N-nitrosoguanidine (NTG) mutagenesis and leucine analog resistant mutants were generated. The mutants showed statistically higher 3H4MV fraction than the parent strain without supplementing leucine. Additionally, by supplying excess amount of leucine, the mutants synthesized 3HB-based PHA copolymer containing 3.1 mol% 3H4MV and 1.2 mol% 3-hydroxyvalerate (3HV) as minor constituents, which significantly affected the thermal properties of the copolymer. This study demonstrates that it is possible to enhance the monomer supply of 3H4MV into PHA by manipulating leucine metabolism.
PMCID: PMC3159905  PMID: 21906338
polyhydroxyalkanoate; copolymer; 3H4MV precursor; leucine analog resistant mutant
23.  Polyhydroxyalkanoate (PHA) Accumulation in Sulfate-Reducing Bacteria and Identification of a Class III PHA Synthase (PhaEC) in Desulfococcus multivorans 
Seven strains of sulfate-reducing bacteria (SRB) were tested for the accumulation of polyhydroxyalkanoates (PHAs). During growth with benzoate Desulfonema magnum accumulated large amounts of poly(3-hydroxybutyrate) [poly(3HB)]. Desulfosarcina variabilis (during growth with benzoate), Desulfobotulus sapovorans (during growth with caproate), and Desulfobacterium autotrophicum (during growth with caproate) accumulated poly(3HB) that accounted for 20 to 43% of cell dry matter. Desulfobotulus sapovorans and Desulfobacterium autotrophicum also synthesized copolyesters consisting of 3-hydroxybutyrate and 3-hydroxyvalerate when valerate was used as the growth substrate. Desulfovibrio vulgaris and Desulfotalea psychrophila were the only SRB tested in which PHAs were not detected. When total DNA isolated from Desulfococcus multivorans and specific primers deduced from highly conserved regions of known PHA synthases (PhaC) were used, a PCR product homologous to the central region of class III PHA synthases was obtained. The complete pha locus of Desulfococcus multivorans was subsequently obtained by inverse PCR, and it contained adjacent phaEDm and phaCDm genes. PhaCDm and PhaEDm were composed of 371 and 306 amino acid residues and showed up to 49 or 23% amino acid identity to the corresponding subunits of other class III PHA synthases. Constructs of phaCDm alone (pBBRMCS-2::phaCDm) and of phaEDmCDm (pBBRMCS-2::phaEDmCDm) in various vectors were obtained and transferred to several strains of Escherichia coli, as well as to the PHA-negative mutants PHB−4 and GPp104 of Ralstonia eutropha and Pseudomonas putida, respectively. In cells of the recombinant strains harboring phaEDmCDm small but significant amounts (up to 1.7% of cell dry matter) of poly(3HB) and of PHA synthase activity (up to 1.5 U/mg protein) were detected. This indicated that the cloned genes encode functionally active proteins. Hybrid synthases consisting of PhaCDm and PhaE of Thiococcus pfennigii or Synechocystis sp. strain PCC 6308 were also constructed and were shown to be functionally active.
PMCID: PMC492432  PMID: 15294771
24.  Biochemical and molecular characterization of the Pseudomonas lemoignei polyhydroxyalkanoate depolymerase system. 
Journal of Bacteriology  1995;177(3):596-607.
Pseudomonas lemoignei has five different polyhydroxyalkanoate (PHA) depolymerase genes (phaZ1 to phaZ5), which encode the extracellularly localized poly(3-hydroxybutyrate) (PHB) depolymerases C, B, and D, poly(3-hydroxyvalerate) (PHV) depolymerase, and PHB depolymerase A, respectively. Four of the five genes (phaZ1 to phaZ4) have been cloned, and one of them (phaZ1) was studied in detail earlier (D. Jendrossek, B. Müller, and H. G. Schlegel, Eur. J. Biochem. 218:701-710, 1993). The fifth PHA depolymerase gene (phaZ5) was identified by colony hybridization of recombinant Escherichia coli clones with a phaZ5-specific oligonucleotide. The nucleotide sequence of a 3,704-bp EcoRI fragment was determined and found to contain two large open reading frames (ORFs) which coded for a polypeptide with significant similarities to glycerol-3-phosphate dehydrogenases of various sources (313 amino acids; M(r), 32,193) and for the precursor of PHB depolymerase A (PhaZ5; 433 amino acids; M(r), 44,906). The PHV depolymerase gene (phaZ4) was subcloned, and the nucleotide sequence of a 3,109-bp BamHI fragment was determined. Two large ORFs (ORF3 and ORF4) that represent putative coding regions were identified. The deduced amino acid sequence of ORF3 (134 amino acids; M(r), 14,686) revealed significant similarities to the branched-chain amino acid aminotransferase (IlfE) of enterobacteria. ORF4 (1,712 bp) was identified as the precursor of a PHV depolymerase (567 amino acids; M(r), 59,947). Analysis of primary structures of the five PHA depolymerases of P. lemoignei and of the PHB depolymerases of Alcaligenes faecalis and Pseudomonas pickettii revealed homologies of 25 to 83% to each other and a domain structure: at their N termini, they have typical signal peptides of exoenzymes. The adjacent catalytic domains are characterized by several conserved amino acids that constitute putative catalytic triads which consist of the consensus sequence of serine-dependent hydrolases including the pentapeptide G-X-S-X-G, a conserved histidine and aspartate, and a conserved region resembling the oxyanion hole of lipases. C terminal of the catalytic domain an approximately 40-amino-acid-long threonine-rich region (22 to 27 threonine residues) is present in PhaZ1, PhaZ2, PhaZ3, and PhaZ5. Instead of the threonine-rich region PhaZ4 and the PHB depolymerases of A. faecalis and P. pickettii contain an approximately 90-amino-acid-long sequence resembling the fibronectin type III module of eucaryotic extracellular matrix proteins. The function of the fibronectin type III module in PHA depolymerases remains obscure. Two types of C-terminal sequences apparently represent substrate-binding sites; the PHB type is present in the PHB depolymerases of A. faecalis and P. pickettii and in PhaZ2, PhaZ3, and PhaZ5 and the PHV type is present in the PHV-hydrolyzing depolymerases (PhaZ4 and PhaZ1). phaZ1 was transferred to A. eutrophus H16 and JMP222. All transconjugants of both strains were able to grow with extracellular PHB as a carbon source and produced translucent halos on PHB-containing solid media. PhaZ1, PhaZ2, PhaZ4, and PhaZ5 were purified from P. lemoignei and from recombinant E. coli; the processing sites of the precursors in E. coli were the same as in P. lemoignei, and similar substrate specificities were determined for the wild-type and the recombinant proteins. All PHA depolymerases hydrolyzed PHB at high specific activities. PhaZ1 and PhaZ4 additionally cleaved PHV, and PhaZ4 hydrolyzed poly(4-hydroxybutyrate). None of the depolymerases was able to hydrolyze polyactide or PHA consisting of monomers with more than five carbon atoms. While the wild-type depolymerase proteins were glycosylated and found to contain glucose and N-acetylglucosamine, none of the recombinant proteins was glycosylated. PHB hydrolysis was dependent on divalent cations such as Ca2+ and was inhibited by the presence of EDTA.
PMCID: PMC176633  PMID: 7836292
25.  Sequence analysis and structure prediction of type II Pseudomonas sp. USM 4–55 PHA synthase and an insight into its catalytic mechanism 
Polyhydroxyalkanoates (PHA), are biodegradable polyesters derived from many microorganisms such as the pseudomonads. These polyesters are in great demand especially in the packaging industries, the medical line as well as the paint industries. The enzyme responsible in catalyzing the formation of PHA is PHA synthase. Due to the limited structural information, its functional properties including catalysis are lacking. Therefore, this study seeks to investigate the structural properties as well as its catalytic mechanism by predicting the three-dimensional (3D) model of the Type II Pseudomonas sp. USM 4–55 PHA synthase 1 (PhaC1P.sp USM 4–55).
Sequence analysis demonstrated that PhaC1P.sp USM 4–55 lacked similarity with all known structures in databases. PSI-BLAST and HMM Superfamily analyses demonstrated that this enzyme belongs to the alpha/beta hydrolase fold family. Threading approach revealed that the most suitable template to use was the human gastric lipase (PDB ID: 1HLG). The superimposition of the predicted PhaC1P.sp USM 4–55 model with 1HLG covering 86.2% of the backbone atoms showed an RMSD of 1.15 Å. The catalytic residues comprising of Cys296, Asp451 and His479 were found to be conserved and located adjacent to each other. In addition to this, an extension to the catalytic mechanism was also proposed whereby two tetrahedral intermediates were believed to form during the PHA biosynthesis. These transition state intermediates were further postulated to be stabilized by the formation of oxyanion holes. Based on the sequence analysis and the deduced model, Ser297 was postulated to contribute to the formation of the oxyanion hole.
The 3D model of the core region of PhaC1P.sp USM 4–55 from residue 267 to residue 484 was developed using computational techniques and the locations of the catalytic residues were identified. Results from this study for the first time highlighted Ser297 potentially playing an important role in the enzyme's catalytic mechanism.
PMCID: PMC1636056  PMID: 17076907

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