REGULATION OF PHB METABOLISM IN ALCALIGENES-EUTROPHUS

Metabolites associated with the poly(3-hydroxybutyrate) (PHB) biospnthetic pathway in Alcaligenes eutrophus were measured to gain an insight into the regulation of PHB synthesis in vivo. Alcaligenes eutrophus was grown in carbon-limited chemostat culture to provide bacteria producing negligible PHB, and in nitrogen-limited chemostat culture to yield PHB-synthesizing bacteria. 3-Hydroxybutyryl-CoA (3HBCoA) was detected only in polymer-accumulating bacteria. The level of coenzyme A (CoASH) was approximately three times higher in the absence of PHB synthesis, in accord with the putative role of this metabolite in the regulation of 3-ketothiolase. The level of acetoacetyl-CoA was, however, similar in PHB-accumulating and nonaccumulating bacteria, suggesting that NADPH-acetoacetyl-CoA reductase may regulate PHB synthesis in bacteria grown under carbon limitation. Immediately after nitrogen exhaustion in batch culture of A. eutrophus, there was an initial large decrease in the weight-average molecular weight, which corresponded to the rapid disappearance of CoASH and the maximum level of 3HBCoA. The decrease in the rate of PHB synthesis in batch culture was consistent with regulation involving NADPH-acetoacetyl-CoA reductase. The disappearance of 3HBCoA coincided with the cessation of PHB synthesis and the maximum level of acetyl-CoA.