Production of two phase polyhydroxyalkanoic acid granules in Ralstonia eutropha

Phosphorus limitation strategy to increase propionic acid flux towards 3-hydroxyvaleric acid monomers in Cupriavidus necator

Properties of polyhydroxybutyrate-co-hydroxyvalerate (P(3HB-co-3HV)) depend on their 3HV content. 3HV can be produced by Cupriavidus necator from propionic acid. Few studies explored carbon distribution and dynamics of 3HV and 3HB monomers production, and none of them have been done with phosphorus as limiting nutrient. In this study, fed-batch cultures of C. necator with propionic acid, as sole carbon source or mixed with butyric acid, were performed. Phosphorus deficiency allowed sustaining 3HV production rate and decreasing 3HB production rate, leading to an instant production of up to 100% of 3HV. When a residual growth is sustained by a phosphorus feeding, the maximum 3HV percentage produced from propionic acid is limited to 33% (Mole.Mole(-1)). The association of a second carbon source like butyric acid lead to higher conversion of propionic acid into 3HV. This study showed the importance of the limiting nutrient and of the culture strategy to get the appropriate product.

Expression of Aeromonas caviae polyhydroxyalkanoate synthase gene in Burkholderia sp. USM (JCM15050) enables the biosynthesis of SCL-MCL PHA from palm oil products

AIMS

Burkholderia sp. USM (JCM15050) isolated from oil-polluted wastewater is capable of utilizing palm oil products and glycerol to synthesize poly(3-hydroxybutyrate) [P(3HB)]. To confer the ability to produce polymer containing 3-hydroxyhexanoate (3HHx), plasmid (pBBREE32d13) harbouring the polyhydroxyalkanoate (PHA) synthase gene of Aeromonas caviae (phaC(Ac)) was transformed into this strain.

METHODS AND RESULTS

  The resulting transformant incorporated approximately 1 ± 0·3 mol% of 3HHx in the polymer when crude palm kernel oil (CPKO) or palm kernel acid oil was used as the sole carbon source. In addition, when the transformed strain was cultivated in the mixtures of CPKO and sodium valerate, PHA containing 69 mol% 3HB, 30 mol% 3-hydroxyvalerate and 1 mol% 3HHx monomers was produced. Batch feeding of carbon sources with 0·5% (v/v) CPKO at 0 h and 0·25% (w/v) sodium valerate at 36 h yielded 6 mol% of 3HHx monomer by controlled-feeding strategies.

CONCLUSIONS

Burkholderia sp. USM (JCM15050) has the metabolic pathways to supply both the short-chain length (SCL) and medium-chain length (MCL) PHA monomers. By transforming the strain with the Aer. caviae PHA synthase with broader substrate specificity, SCL-MCL PHA was produced.

SIGNIFICANCE AND IMPACT OF THE STUDY

  This is the first study demonstrating the ability of transformant Burkholderia to produce P(3HB-co-3HHx) from a single carbon source.

Effects of recombinant precursor pathway variations on poly[(R)-3-hydroxybutyrate] synthesis in Saccharomyces cerevisiae

Different recombinant R-3-hydroxybutyryl-CoA (3-HB) synthesis pathways strongly influenced the rate and accumulation of the biopolymer poly[(R)-3-hydroxybutyrate] (PHB) in Saccharomyces cerevisiae. It has been previously shown that expression of the Cupriavidus necator PHB synthase gene leads to PHB accumulation in S. cerevisiae [Leaf, T., Peterson, M., Stoup, S., Somers, D., Srienc, F., 1996. Saccharomyces cerevisiae expressing bacterial polyhydroxybutyrate synthase produces poly-3-hydroxybutyrate. Microbiology 142, 1169-1180]. This finding indicates that native S. cerevisiae expresses genes capable of synthesizing the correct stereochemical substrate for the synthase enzyme. The effects of variations of 3-HB precursor pathways on PHB accumulation were investigated by expressing combinations of C. necator PHB pathway genes. When only the PHB synthase gene was expressed, the cells accumulated biopolymer to approximately 0.2% of their cell dry weight. When the PHB synthase and reductase gene were co-expressed, the PHB levels increased approximately 18 fold to about 3.5% of the cell dry weight. When the beta-ketothiolase, reductase and synthase genes were all expressed, the strain accumulated PHB to approximately 9% of the cell dry weight which is 45 fold higher than in the strain with only the synthase gene. Fluorescent microscopic analysis revealed significant cell-to-cell heterogeneity in biopolymer accumulation. While the population average for the strain expressing three PHB genes was approximately 9% of the cell dry weight, some cells accumulated PHB in excess of 50% of their cell volume. Other cells accumulated no biopolymer. In addition, the recombinant strain was shown to co-produce ethanol and PHB under anaerobic conditions. These results demonstrate that the technologically important organism S. cerevisiae is capable of accumulating PHB aerobically and anaerobically at levels similar to some bacterial systems. The easily assayed PHB system also creates a convenient means of probing in vivo the presence of intracellular metabolites which could be useful for studying the intermediary metabolism of S. cerevisiae.

Detection of intermediates from the polymerization reaction catalyzed by a D302A mutant of class III polyhydroxyalkanoate (PHA) synthase

Polyhydroxybutyrate (PHB) synthases catalyze the polymerization of (R)-3-hydroxybutyryl-CoA (HB-CoA) into high molecular weight PHB, biodegradable polymers. The class III PHB synthase from Allochromatium vinosum is composed of a 1:1 mixture of two approximately 40 kDa proteins: PhaC and PhaE. Previous studies using site-directed mutagenesis and a saturated trimer of hydroxybutyryl-CoA have suggested the importance of C149 (in covalent catalysis), H331 (in activation of C149), and D302 (in hydroxyl group activation for ester bond formation) in the polymerization process. All three residues are located on PhaC. We now report that incubation of D302A-PhaCPhaE with [14C]-HB-CoA results in detection, for the first time, of oligomeric HBs covalently bound to PhaC. The reaction products have been analyzed by SDS-PAGE, Westerns with PhaCPhaE antibodies, and autoradiography. Different migratory properties of D302A-PhaC on SDS-PAGE have been observed at [14C]-HB-CoA to enzyme (S/E) ratios between 5 and 100. Trypsin digestion and HPLC analysis of the D302A-PhaCPhaE (from a reaction with a S/E ratio of 5) allowed isolation of multiple radiolabeled peptides. N-Terminal sequencing, MALDI-TOF, and ESI mass spectrometric analysis of these peptides revealed that all of the peptides were identical but were modified by (HB)n ranging in size from n = 3 to n = 10. The in vitro results support the role of D302 in elongation rather than in activating the active site cysteine for acylation. This proposal has been further supported by our in vivo studies on a Wautersia eutropha strain in which the class I synthase gene has been replaced with the D302A-PhaCPhaE gene and the organism examined under PHB production conditions by transmission electron microscopy. Very small granules (<0.05 microm) were observed in contrast to the 0.2-0.5 microm granules observed with the wt strain. Use of the D302A synthase has allowed successful interrogation of the initiation and elongation steps catalyzed by the class III synthase.

Kinetic studies and biochemical pathway analysis of anaerobic poly-(R)-3-hydroxybutyric acid synthesis in Escherichia coli

Poly-(R)-3-hydroxybutyric acid (PHB) was synthesized anaerobically in recombinant Escherichia coli. The host anaerobically accumulated PHB to more than 50% of its cell dry weight during cultivation in either growth or nongrowth medium. The maximum specific PHB production rate during growth-associated synthesis was approximately 2.3 +/- 0.2 mmol of PHB/g of residual cell dry weight/h. The by-product secretion profiles differed significantly between the PHB-synthesizing strain and the control strain. PHB production decreased acetate accumulation for both growth and nongrowth-associated PHB synthesis. For instance under nongrowth cultivation, the PHB-synthesizing culture produced approximately 66% less acetate on a glucose yield basis as compared to a control culture. A theoretical biochemical network model was used to provide a rational basis to interpret the experimental results like the fermentation product secretion profiles and to study E. coli network capabilities under anaerobic conditions. For example, the maximum theoretical carbon yield for anaerobic PHB synthesis in E. coli is 0.8. The presented study is expected to be generally useful for analyzing, interpreting, and engineering cellular metabolisms.

Biochemical observations on medium-chain-length polyhydroxyalkanoate biosynthesis and accumulation in Pseudomonas mendocina

Certain Pseudomonads are capable of accumulating high levels of medium-chain-length polyhydroxyalkanates (PHAmcl) when grown with carbohydrates as the main carbon source. 3-OH acyl components of PHAmcl are derived from fatty acid synthase (FAS) and these components are accessed by action of 3-hydroxyacyl-acyl carrier protein (ACP)-coenzyme A (CoA) transferase (transacylase). However, little is known with regard to the time courses of 3-OH acyl component occurrence and of transacylase activity during PHAmcl induction. Also, little is known with regard to the coupling mechanism between FAS and PHAmcl synthesis or whether the FAS pathway itself is specialized in PHAmcl-producing cells. Our results with regard to the time course of formation of 3-OH acids, 3-OH acyl-ACPs, and PHAmcl are consistent with the view that transacylase provides the key link between FAS and PHAmcl synthase. They also suggest that FAS specialization is not a feature of the mechanism. Further, we observed the formation of a 3-OH 10:0 homopolymer early in the induction phase followed by later formation of a mixed polymer containing 3-OH 8:0 and 3-OH 12:0 in addition to 3-OH 10:0. Early occurrence of 3-OH 10:0-CoA transacylase activity was coincident with homopolymer formation.

Biochemical and molecular basis of microbial synthesis of polyhydroxyalkanoates in microorganisms

Intensive research on the physiology, biochemistry, and molecular genetics of the metabolism of polyhydroxyalkanoates (PHA) during the last 15 years has revealed a dramatic increase of our knowledge on the biosynthesis of these polyesters in bacteria. This mainly very basic research has revealed several new, hitherto not described enzymes and pathways. In addition, many genes encoding the enzymes of these pathways and in particular the key enzyme of PHA biosynthesis, PHA synthase, were cloned and characterized at a molecular level. This knowledge was utilized to establish PHA biosynthesis in many prokaryotic and eukaryotic organisms, which were unable to synthesize PHAs, and to apply the methodology of metabolic engineering, thus opening new perspectives for the production of various PHAs by fermentation biotechnology or agriculture in economically feasible processes. This contribution summarizes the properties of PHA synthases and gives an overview on the genes for these enzymes and other enzymes of PHA biosynthesis that have been cloned and are available. It also summarizes our current knowledge on the regulation at the enzyme and gene level of PHA biosynthesis in bacteria.