Regulation, sensory domains and roles of two Desulfovibrio desulfuricans ATCC27774 Crp family transcription factors, HcpR1 and HcpR2, in response to nitrosative stress

In silico analyses identified a Crp/Fnr family transcription factor (HcpR) in sulfate-reducing bacteria that controls expression of the hcp gene, which encodes the hybrid cluster protein and contributes to nitrosative stress responses. There is only one hcpR gene in the model sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough, but two copies in Desulfovibrio desulfuricans 27774, which can use nitrate as an alternative electron acceptor to sulfate. Structures of the D. desulfuricans hcpR1, hcpR2 and hcp operons are reported. We present evidence that hcp expression is regulated by HcpR2, not by HcpR1, and that these two regulators differ in both their DNA-binding site specificity and their sensory domains. HcpR1 is predicted to be a b-type cytochrome. HcpR1 binds upstream of the hcpR1 operon and its synthesis is regulated coordinately with hcp in response to NO. In contrast, hcpR2 expression was not induced by nitrate, nitrite or NO. HcpR2 is an iron-sulfur protein that reacts with NO and O2 . We propose that HcpR1 and HcpR2 use different sensory mechanisms to regulate subsets of genes required for defense against NO-induced nitrosative stress, and that diversification of signal perception and DNA recognition by these two proteins is a product of D. desulfuricans adaptation to its particular environmental niche.