The bzd gene cluster, coding for anaerobic benzoate catabolism, in Azoarcus sp. strain CIB

We report here that the bzd genes for anaerobic benzoate degradation in Azoarcus sp. strain CIB are organized as two transcriptional units, i.e., a benzoate-inducible catabolic operon, bzdNOPQMSTUVWXYZA, and a gene, bzdR, encoding a putative transcriptional regulator. The last gene of the catabolic operon, bzdA, has been expressed in Escherichia coli and encodes the benzoate-coenzyme A (CoA) ligase that catalyzes the first step in the benzoate degradation pathway. The BzdA enzyme is able to activate a wider range of aromatic compounds than that reported for other previously characterized benzoate-CoA ligases. The reduction of benzoyl-CoA to a nonaromatic cyclic intermediate is carried out by a benzoyl-CoA reductase (bzdNOPQ gene products) detected in Azoarcus sp. strain CIB extracts. The bzdW, bzdX, and bzdY gene products show significant similarity to the hydratase, dehydrogenase, and ring-cleavage hydrolase that act sequentially on the product of the benzoyl-CoA reductase in the benzoate catabolic pathway of Thauera aromatica. Benzoate-CoA ligase assays and transcriptional analyses based on lacZ-reporter fusions revealed that benzoate degradation in Azoarcus sp. strain CIB is subject to carbon catabolite repression by some organic acids, indicating the existence of a physiological control that connects the expression of the bzd genes to the metabolic status of the cell.