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.