13C nuclear magnetic resonance studies of Pseudomonas putida fatty acid metabolic routes involved in poly(3-hydroxyalkanoate) synthesis

Engineering of stable recombinant bacteria for production of chiral medium-chain-length poly-3-hydroxyalkanoates

In order to scale up medium-chain-length polyhydroxyalkanoate (mcl-PHA) production in recombinant microorganisms, we generated and investigated different recombinant bacteria containing a stable regulated expression system for phaC1, which encodes one of the mcl-PHA polymerases of Pseudomonas oleovorans. We used the mini-Tn5 system as a tool to construct Escherichia coli 193MC1 and P. oleovorans POMC1, which had stable antibiotic resistance and PHA production phenotypes when they were cultured in a bioreactor in the absence of antibiotic selection. The molecular weight and the polydispersity index of the polymer varied, depending on the inducer level. E. coli 193MC1 produced considerably shorter polyesters than P. oleovorans produced; the weight average molecular weight ranged from 67,000 to 70,000, and the polydispersity index was 2.7. Lower amounts of inducer added to the media shifted the molecular weight to a higher value and resulted in a broader molecular mass distribution. In addition, we found that E. coli 193MC1 incorporated exclusively the R configuration of the 3-hydroxyoctanoate monomer into the polymer, which corroborated the enantioselectivity of the PhaC1 polymerase enzyme.

Co-expression of 3-ketoacyl-ACP reductase and polyhydroxyalkanoate synthase genes induces PHA production in Escherichia coli HB101 strain

The Escherichia coli 3-ketoacyl-ACP reductase gene (fabGEc) was cloned using a PCR technique to investigate the metabolic link between fatty acid metabolism and polyhydroxyalkanoate (PHA) production. Three plasmids respectively harboring fabGEc and the poly-3-hydroxyalkanoate synthesis genes phaCAc and phaC1Ps from Aeromonas caviae and Pseudomonas sp. 61-3 respectively were constructed and introduced into E. coli HB101 strain. On a two-stage cultivation using dodecanoate as the sole carbon source, recombinant E. coli HB101 strains harboring fabGEc and phaC genes accumulated PHA copolymers (about 8 wt% of dry cell weight) consisting of several (R)-3-hydroxyalkanoate units of C4, C6, C8, and C10. It has been suggested that overexpression of the fabGEc gene leads to the supply of (R)-3-hydroxyacyl-CoA for PHA synthesis via fatty acid degradation.

Development of environmentally friendly coatings and paints using medium-chain-length poly(3-hydroxyalkanoates) as the polymer binder

Unsaturated medium-chain-length poly(3-hydroxyalkanoates) (mcl-PHAs) produced by Pseudomonas putida from linseed oil fatty acids (LOFA) and tall oil fatty acids (TOFA), were used as the polymer binder in the formulation of high solid alkyd-like paints. The relatively high concentration of unsaturated alkyl side chains incorporated into the PHA resins resulted in oxidative drying PHA paints having excellent coating properties. The homogeneously pigmented PHA coatings yielded high-gloss, smooth and strong films upon curing and showed an excellent flexibility, a good adhesion to different substrates, cohesive film properties and resistance to chipping.

Perspectives of medium chain length poly(hydroxyalkanoates), a versatile set of bacterial bioplastics

Medium chain length (mcl) poly(hydroxyalkanoic acids) (PHAs) are polyesters accumulated by fluorescent Pseudomonads and other bacteria. Work on the genetics of mcl-PHA formation has led to polymer synthesis in recombinant bacteria and plants. Several high and medium cost applications are now emerging. With optimized bacterial mcl-PHA synthesis on inexpensive agro-substrates and the development of plant-based mcl-PHAs in the next decade, the production economics of these bioplastics will ultimately permit their sustainable production for bulk applications.

Production of medium-chain-length poly(3-hydroxyalkanoates) from gluconate by recombinant Escherichia coli

It was shown recently that recombinant Escherichia coli, defective in the beta-oxidation cycle and harboring a medium-chain-length (MCL) poly(3-hydroxyalkanoate) (PHA) polymerase-encoding gene of Pseudomonas, is able to produce MCL PHA from fatty acids but not from sugars or gluconate (S. Langenbach, B. H. A. Rehm, and A. Steinbüchel, FEMS Microbiol. Lett. 150:303-309, 1997; Q. Ren, Ph.D. thesis, ETH Zürich, Zürich, Switzerland, 1997). In this study, we report the formation of MCL PHA from gluconate by recombinant E. coli. By introduction of genes coding for an MCL PHA polymerase and the cytosolic thioesterase I ('thioesterase I) into E. coli JMU193, we were able to engineer a pathway for the synthesis of MCL PHA from gluconate. We used two expression systems, i.e., the bad promoter and alk promoter, for the 'thioesterase I- and PHA polymerase-encoding genes, respectively, which enabled us to modulate their expression independently over a range of inducer concentrations, which resulted in a maximum MCL PHA accumulation of 2.3% of cell dry weight from gluconate. We found that the amount of PHA and the 'thioesterase I activity are directly correlated. Moreover, the polymer accumulated in the recombinant E. coli consisted mainly of 3-hydroxyoctanoate monomers. On the basis of our data, we propose an MCL PHA biosynthesis pathway scheme for recombinant E. coli JMU193, harboring PHA polymerase and 'thioesterase I, when grown on gluconate, which involves both de novo fatty acid synthesis and beta-oxidation.

Synthesis of medium-chain-length polyhydroxyalkanoates in arabidopsis thaliana using intermediates of peroxisomal fatty acid beta-oxidation

Polyhydroxyalkanoate (PHA) is a family of polymers composed primarily of R-3-hydroxyalkanoic acids. These polymers have properties of biodegradable thermoplastics and elastomers. Medium-chain-length PHAs (MCL-PHAs) are synthesized in bacteria by using intermediates of the beta-oxidation of alkanoic acids. To assess the feasibility of producing MCL-PHAs in plants, Arabidopsis thaliana was transformed with the PhaC1 synthase from Pseudomonas aeruginosa modified for peroxisome targeting by addition of the carboxyl 34 amino acids from the Brassica napus isocitrate lyase. Immunocytochemistry demonstrated that the modified PHA synthase was appropriately targeted to leaf-type peroxisomes in light-grown plants and glyoxysomes in dark-grown plants. Plants expressing the PHA synthase accumulated electron-lucent inclusions in the glyoxysomes and leaf-type peroxisomes, as well as in the vacuole. These inclusions were similar to bacterial PHA inclusions. Analysis of plant extracts by GC and mass spectrometry demonstrated the presence of MCL-PHA in transgenic plants to approximately 4 mg per g of dry weight. The plant PHA contained saturated and unsaturated 3-hydroxyalkanoic acids ranging from six to 16 carbons with 41% of the monomers being 3-hydroxyoctanoic acid and 3-hydroxyoctenoic acid. These results indicate that the beta-oxidation of plant fatty acids can generate a broad range of R-3-hydroxyacyl-CoA intermediates that can be used to synthesize MCL-PHAs.

Metabolic routing towards polyhydroxyalkanoic acid synthesis in recombinant Escherichia coli (fadR): inhibition of fatty acid beta-oxidation by acrylic acid

Heterologous expression of the phaC1 gene from Pseudomonas aeruginosa, which encodes one of the polyhydroxyalkanoic acid synthases, in Escherichia coli impaired in fatty acid beta-oxidation results in polyhydroxyalkanoic acid accumulation when cells were cultivated on fatty acids. We evaluated the application of the fatty acid beta-oxidation inhibitor acrylic acid as a tool to channel intermediates of beta-oxidation to polyhydroxyalkanoic acid synthesis. Various E. coli strains affected in fatty acid metabolism and the wild-type strain harboring plasmid pBHR71 were analyzed with respect to polyhydroxyalkanoic acid accumulation in the presence of acrylic acid. The E. coli fadR mutant RS3097 revealed the strongest polyhydroxyalkanoic acid accumulation. The optimum inhibitory concentration of acrylic acid was 0.24 mg ml-1 and caused efficient channeling of intermediates of beta-oxidation to polyhydroxyalkanoic acid synthesis. Under these conditions and grown on decanoate E. coli RS3097 harboring plasmid pBHR71 revealed a polyhydroxyalkanoic acid accumulation contributing to about 60% of cellular dry weight.

Bacterial and other biological systems for polyester production

Poly(3-hydroxybutyric acid) and other structurally related aliphatic polyesters from bacteria, referred to as polyhydroxyalkanoic acids, form biodegradable thermoplastics and elastomers that are currently in use, or being considered for use, in industry, medicine, pharmacy and agriculture. At present, they are produced by microbial fermentations; in the future, production will also be possible by in vitro methods or by agriculture using transgenic plants. Representatives from this highly diverse class of polyesters might be produced as commodity chemicals for bulk applications, and others as fine chemicals for special applications.

A new metabolic link between fatty acid de novo synthesis and polyhydroxyalkanoic acid synthesis. The PHAG gene from Pseudomonas putida KT2440 encodes a 3-hydroxyacyl-acyl carrier protein-coenzyme a transferase

To investigate the metabolic link between fatty acid de novo synthesis and polyhydroxyalkanoic acid (PHA) synthesis, we isolated mutants of Pseudomonas putida KT2440 deficient in this metabolic route. The gene phaG was cloned by phenotypic complementation of these mutants; it encoded a protein of 295 amino acids with a molecular mass of 33,876 Da, and the amino acid sequence exhibited 44% amino acid identity to the primary structure of the rhlA gene product, which is involved in the rhamnolipid biosynthesis in Pseudomonas aeruginosa PG201. S1 nuclease protection assay identified the transcriptional start site 239 base pairs upstream of the putative translational start codon. Transcriptional induction of phaG was observed when gluconate was provided, and PHA synthesis occurred from this carbon source. No complementation of the rhlA mutant P. aeruginosa UO299-harboring plasmid pBHR81, expressing phaG gene under lac promoter control, was obtained. Heterologous expression of phaG in Pseudomonas oleovorans, which is not capable of PHA synthesis from gluconate, enabled PHA synthesis on gluconate as the carbon source. Native recombinant PhaG was purified by native polyacrylamide gel electrophoresis from P. oleovorans-harboring plasmid pBHR81. It catalyzes the transfer of the acyl moiety from in vitro synthesized 3-hydroxydecanoyl-CoA to acyl carrier protein, indicating that PhaG exhibits a 3-hydroxyacyl-CoA-acyl carrier protein transferase activity.

Synthesis of poly(3-hydroxyalkanoates) in Escherichia coli expressing the PHA synthase gene phaC2 from Pseudomonas aeruginosa: comparison of PhaC1 and PhaC2

In order to obtain functional expression of PHA synthase gene phaC2 from Pseudomonas aeruginosa in Escherichia coli, the coding region of phaC2 was subcloned, including the ribosomal binding site, into pBluescript SK- collinear to the lac promoter. This plasmid pBHR71-C2 enabled functional expression of phaC2 in E. coli LS1298 (fadB) under lac promoter control, leading to PHA accumulation, when grown in LB medium containing 0.5% (w/v) of various fatty acids (C8-C14). The strongest accumulation of PHA was observed, when dodecanoate was provided as carbon source, and PHA contributed to 15% of cell dry weight, which was composed of 35 mol% 3-hydroxydodecanoate, 60 mol% 3-hydroxydecanoate and 5 mol% 3-hydroxyoctanoate. Plasmid pBHR78, which contained both genes phaC1 and phaC2 from P. aeruginosa under lac promoter control in pBluescript SK- led in E. coli LS1298 to PHA accumulation, which contributed to 13% of cell dry weight, when cells were grown on decanoate. Only slight differences in PHA composition compared with either PhaC1 or PhaC2 were obtained. The weight average molecular masses of PHA purified from decanoate-grown cells of E. coli LS1298 expressing PhaC1 or PhaC2 alone or both PHA synthases, were 106 x 10(3), 70 x 10(3) or 67 x 10(3), respectively. This study clearly demonstrated that both PHA synthases from P. aeruginosa exhibit very similar properties resulting in similar extent of PHA accumulation, similar composition and molecular mass, when expressed in E. coli and that fatty acid beta-oxidation provides substrates for both PHA synthases.

In vitro activities of granule-bound poly[(R)-3-hydroxyalkanoate]polymerase C1 of Pseudomonas oleovorans--development of an activity test for medium-chain-length-poly(3-hydroxyalkanoate) polymerases

A newly developed in vitro activity assay for medium-chain-length (mcl)-poly(3-hydroxyalkanoate) polymerases is described. Polymerase C1 of Pseudomonas oleovorans GPo1 attached to isolated granules was used as model enzyme. A direct correlation was found between (R)-3-hydroxyoctanoylCoA depletion and poly(3-hydroxyalkanoate) synthesis due to polymerase C1 activity. Highest activities of 1.13 U/mg granule bound protein and highest specific activities of 2.3 U/mg polymerase C1 were determined towards (RS)-3-hydroxyoctanoylCoA. A first determination of a Km value for mcl poly(3-hydroxyalkanoate) polymerases was performed leading to an estimated Km of 0.16 (+/-0.1) mM for granule bound polymerase C1 with (R)-3-hydroxyoctanoylCoA as substrate. Polymerase C1 showed no activity towards (RS)-3-hydroxybutyrylCoA and a specific activity of 0.28 U/mg polymerase C1 for (R)-3-hydroxyvalerylCoA. (R)-3-HydroxyoctanoylCoA and a mixture of (RS)-3-hydroxyoctanoylCoA were both depleted for more than 75% by granule-bound polymerase C1, suggesting a non-rate-limiting epimerase activity attached to poly(3-hydroxyalkanoate) granules isolated from Pseudomonas putida GPp104[pGEc405]. Whereas no relationship was found between the activity of granule-bound polymerase C1 and poly(3-hydroxyalkanoate) content of the granules, higher activities were measured when a higher substrate concentration or more enzyme was present in the in vitro activity assay.

Polymerase C1 levels and poly(R-3-hydroxyalkanoate) synthesis in wild-type and recombinant Pseudomonas strains

A functional antibody highly specific for polymerase C1 of Pseudomonas oleovorans GPo1 was raised and used to determine polymerase C1 levels in in vivo experiments. The polymerase C1 antibodies did not show a cross-reaction with polymerase C2 of P. oleovorans. In wild-type P. oleovorans GPo1 and Pseudomonas putida KT2442, amounts of 0.075 and 0.06% polymerase relative to total protein, respectively, were found. P. oleovorans GPo1(pGEc405), which contained additional copies of the polymerase C1-encoding gene under the control of its native promoter, contained 0.5% polymerase C1 relative to total protein. Polymerase C1 reached 10% of total cell protein when the polymerase C1-encoding gene was overexpressed through the P(alk) promoter in P. oleovorans GPo1(pET702, pGEc74). Amounts of poly(R-3-hydroxyalkanoate) (PHA) increased significantly under non-nitrogen-limiting conditions when additional polymerase C1 was expressed in P. oleovorans. Whereas P. oleovorans produced 34% (wt/wt) PHA under these conditions, a PHA level of 64% (wt/wt) could be reached for P. oleovorans GPo1(pGEc405) and a PHA level of 52% (wt/wt) could be reached for P. oleovorans GPo1(pET702, pGEc74) after induction, compared to a PHA level of 13% for the uninduced control. All recombinant Pseudomonas strains containing additional polymerase C1 showed small changes in their PHA composition. Larger amounts of 3-hydroxyhexanoate monomer and smaller amounts of 3-hydroxyoctanoate and -decanoate were found compared to those of the wild type. Two different methods were developed to quantify rates of incorporation of new monomers into preexisting PHA granules. P. oleovorans GPo1 cells grown under nitrogen-limiting conditions showed growth stage-dependent incorporation rates. The highest PHA synthesis rates of 9.5 nmol of C8/C6 monomers/mg of cell dry weight (CDW)/min were found during the mid-stationary phase, which equals a rate of production of 80 g of PHA/kg of CDW/h.

Biosynthesis of Poly(3-Hydroxyalkanoic Acid) Copolymer from CO(inf2) in Pseudomonas acidophila through Introduction of the DNA Fragment Responsible for Chemolithoautotrophic Growth of Alcaligenes hydrogenophilus

Pseudomonas acidophila is a bacterial strain producing a poly(3-hydroxyalkanoic acid) (PHA) copolymer from low-molecular-weight organic compounds such as formate and acetate. The genes responsible for PHA production were cloned in cosmid pIK7 containing a 14.8-kb HindIII fragment of P. acidophila DNA. With the aim of developing a means of producing a PHA copolymer from CO(inf2), cosmid pIK7 was introduced into a polymer-negative mutant of the chemolithoautotrophic bacterium Alcaligenes eutrophus PHB(sup-)4. However, the recombinant strain produced a homopolymer of 3-hydroxybutyric acid (polyhydroxybutyric acid) from CO(inf2). Since it was thought that the composition of the accumulated polymer might depend not on the PHA biosynthetic genes but on the metabolism of the host strain, a recombinant plasmid, pFUS, containing the genes for chemolithoautotrophic growth of the hydrogen-oxidizing bacterium A. hydrogenophilus was introduced into P. acidophila by conjugation. The recombinant plasmid pFUS was stably maintained in P. acidophila in the absence of chemolithoautotrophic or antibiotic selection. This pFUS-harboring strain possessed the ability to grow under a gas mixture of H(inf2), O(inf2), and CO(inf2) in a mineral salts medium, and PHA copolymer accumulation was confirmed by nuclear magnetic resonance spectral analysis. A gas chromatogram obtained by gas chromatography-mass spectrometry showed the composition of the polymer to be 52.8% 3-hydroxybutyrate, 41.1% 3-hydroxyoctanoate, and 6.1% 3-hydroxydecanoate. This is the first report of the production of a PHA copolymer from CO(inf2) as sole carbon source.

Physiological responses of Pseudomonas putida KT2442 to phosphate starvation

The physiological responses of Pseudomonas putida KT2442 to phosphate starvation were examined with respect to cell morphology, qualitative demonstration of the accumulation of the intracellular storage component poly-3-hydroxyalkanoate (PHA), cellular ATP and ribosome content, and the rate of total protein synthesis. Upon prolonged incubation under phosphate-limiting conditions, the number of viable cells decreased by two to three orders of magnitude during the first 3 weeks. However, after this decline, viability of the cultures remained remarkably constant for many weeks. The cells remained rod-shaped under phosphate starvation conditions with a tendency to swell in parallel with the accumulation of PHA. Protein synthesis and ribosome concentration were gradually reduced, and ATP levels dropped to very low values after the onset of starvation; later, however, there was a return to near-normal ATP concentrations. Evidence was obtained that the strong selective pressure imposed by phosphate deprivation forces the selection of mutants with a competitive advantage. These mutants are able to grow, possibly utilizing nutrients derived from dead cells, and eventually take over the cultures. One frequently encountered mutant formed smaller colonies on rich solidified medium and displayed an altered cell morphology. This mutant was isolated and further characterized. By employing a bioluminescence-based marker system, we demonstrated that this mutant is able to replace wild-type cells in mixed culture experiments. Thus, long-term phosphate-deprived cultures represent dynamic regimes that can undergo population shifts.

Formation of novel poly(hydroxyalkanoates) from long-chain fatty acids

Poly(hydroxyalkanoates) (PHAs) were isolated from Pseudomonas aeruginosa 44T1 cultivated on euphorbia oil and castor oil. With the aid of 2-D proton NMR spectra and proton-detected multiple bond coherence NMR spectra the structures of the PHAs were determined. In addition to the usual PHA constituents (C6-C14 3-hydroxy fatty acids), PHAs formed from euphorbia oil contained delta 8,9-epoxy-3-hydroxy-5c-tetradecenoate, and probably delta 6,7-epoxy-3-hydroxydodecanoate and delta 4,5-epoxy-3-hydroxydecanoate. These novel constituents account for approximately 15% of the total amount of monomers and are clearly generated via beta-oxidation of vernolic acid (delta 12,13-epoxy-9c-octadecenoic acid), the main component of euphorbia oil. In PHAs formed from castor oil, 7% of the monomers found were derived from ricinoleic acid (12-hydroxy-9c-octadecenoic acid). The presence of 3,8-dihydroxy-5c-tetradecenoate was clearly demonstrated. Furthermore, NMR analysis strongly suggested the presence of 3,6-dihydroxydodecanoate, 6-hydroxy-3c-dodecenoate, and 4-hydroxydecanoate.

Strategies for the sustainable production of new biodegradable polyesters in plants: a review

In this study we review relevant pathways with regard to the production of poly(3-hydroxyalkanoates) (PHA) with medium chain length monomers in higher plants. On the basis of what is known of the genetics and the biochemistry of PHA formation in bacteria, and of fatty acid metabolism in various organisms, a number of possibilities for PHA production in model plants and in economically important crop plants are listed. Along with the molecular biology of PHA synthesis and fatty acid metabolism, we discuss theoretical and environmental considerations, metabolic engineering strategies, and plant transformation systems.Key words: polyhydroxyalkanoate, fatty acid, starch, potato, Arabidopsis.

Production of polyhydroxyalkanoates, a family of biodegradable plastics and elastomers, in bacteria and plants

In response to problems associated with plastic waste and its effect on the environment, there has been considerable interest in the development and production of biodegradable plastics. Polyhydroxyalkanoates (PHAs) are polyesters that accumulate as inclusions in a wide variety of bacteria. These bacterial polymers have properties ranging from stiff and brittle plastics to rubber-like materials. Because of their inherent biodegradability, PHAs are regarded as an attractive source of nonpolluting plastics and elastomers that can be used for specialty and commodity products. The possibility of producing PHAs in large scale and at a cost comparable to synthetic plastics has arisen from the demonstration of PHA accumulation in transgenic Arabidopsis plants expressing the bacterial PHA biosynthetic genes. Synergism between knowledge of the enzymes and genes contributing to PHA synthesis in bacteria and engineering of plant metabolic pathways will be necessary for the development of crop plants that produce biodegradable plastics.

Production of poly(hydroxyalkanoic acid)

Poly(hydroxyalkanoic acid) [PHA] is accumulated by numerous microorganisms as an energy reserve material under unbalanced growth conditions in the presence of excess carbon source. In spite of being a good candidate for biodegradable thermoplastics, their high price compared with conventional plastics currently in use has limited their availability in a wide range of applications. With the aim of reducing the high production cost of PHA, much effort is currently being devoted to improve productivity by employing various microorganisms and by developing efficient culture techniques. Several processes recently developed and employed for the production of PHA by various bacteria are described.