Connection between poly-beta-hydroxybutyrate biosynthesis and growth on C(1) and C(2) compounds in the methylotroph Methylobacterium extorquens AM1

Several DNA regions containing genes involved in poly-beta-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 beta-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 beta-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 beta-hydroxybutyrate. However, the phaA, phaB, and phaC mutants all showed defects in PHB synthesis. Surprisingly, these mutants also showed defects in growth on C(1) and C(2) compounds and, for phaB, these defects were rescued by glyoxylate supplementation. These results suggest that beta-hydroxybutyryl-CoA is an intermediate in the unknown pathway that converts acetyl-CoA to glyoxylate in methylotrophs and Streptomyces spp.