Oxygen-Dependent Regulation of the Central Pathway for the Anaerobic Catabolism of Aromatic Compounds in Azoarcus sp. Strain CIB

摘要

The role of oxygen in the transcriptional regulation of the P_N promoter that controls the bzd operon involved in the anaerobic catabolism of benzoate in the denitrifying Azoarcus sp. strain CIB has been investigated. In vivo experiments using P_N::lacZ translational fusions, in both Azoarcus sp. strain CIB and Escherichia coli cells, have shown an oxygen-dependent repression effect on the transcription of the bzd catabolic genes. E. coli Fnr was required for the anaerobic induction of the P_N promoter, and the oxygen-dependent repression of the bzd genes could be bypassed by the expression of a constitutively active Fnr* protein. In vitro experiments revealed that Fnr binds to the P_N promoter at a consensus sequence centered at position ?41.5 from the transcription start site overlapping the ?35 box, suggesting that P_N belongs to the class II Fnr-dependent promoters. Fnr interacts with RNA polymerase (RNAP) and is strictly required for transcription initiation after formation of the RNAP-P_N complex. An fnr ortholog, the acpR gene, was identified in the genome of Azoarcus sp. strain CIB. The Azoarcus sp. strain CIB acpR mutant was unable to grow anaerobically on aromatic compounds and it did not drive the expression of the P_N::lacZ fusion, suggesting that AcpR is the cognate transcriptional activator of the P_N promoter. Since the lack of AcpR in Azoarcus sp. strain CIB did not affect growth on nonaromatic carbon sources, AcpR can be considered a transcriptional regulator of the Fnr/Crp superfamily that has evolved to specifically control the central pathway for the anaerobic catabolism of aromatic compounds in Azoarcus.