Characterization of genes involved in poly-β-hydroxybutyrate metabolism in Azospirillum brasilense

Under suboptimal growth conditions and like many other prokaryotes, rhizobacteria of the genus Azospirillum produce high levels of poly-β-hydroxybutyrate (PHB). The two genes, bdhA (3-hydroxybutyrate dehydrogenase) and acsA2 (acetoacetyl-CoA synthetase), which are considered to be involved in the PHB degradation pathway in Azospirillum brasilense strain Sp7, were identified, cloned, and sequenced. Additionally, the expression of the bacterial genes phbA (β-ketothiolase) and phbC (PHB synthase), which are involved in PHB biosynthesis and in the expression of the acsA2 gene, were studied using GUS fusions. Our results indicate that these genes are constitutively expressed in Azospirillum brasilense Sp7 and that the Ntr, P_II and P_Z nitrogen regulatory systems, which have been shown to be involved in the regulation of PHB synthesis, do not affect the expression of these genes. Expression of these genes is also shown to occur during association of A. brasilense with wheat roots.