Nitric oxide (NO) is an important signaling molecule that regulates diverse physiological processes in all domains of life. In many gammaproteobacteria, NO controls behavioral responses through a complex signaling network involving heme-nitric oxide/oxygen binding (H-NOX) domains as selective NO sensors. In Shewanella oneidensis, H-NOX-mediated NO sensing increases biofilm formation, which is thought to serve as a protective mechanism against NO cytotoxicity. The H-NOX/NO-responsive (hno) signaling network involves H-NOX-dependent control of HnoK autophosphorylation and phosphotransfer from HnoK to three response regulators. Two of these response regulators, HnoB and HnoD, regulate cyclic-di-GMP levels and influence biofilm formation. However, the role of the third response regulator in the signaling network, HnoC, has not been determined. Here we describe a role for HnoC as a transcriptional repressor for the signaling genes in the hno network. The genes controlled by HnoC were identified by microarray analysis, and its function as a repressor was confirmed in vivo. HnoC belongs to an uncharacterized family of DNA-binding response regulators. Binding of HnoC to its promoter targets was characterized in vitro, revealing an unprecedented regulation mechanism, which further extends the functional capabilities of DNA-binding response regulators. In the unphosphorylated state HnoC forms a tetramer, which tightly binds to an inverted-repeat target sequence overlapping with the promoter regions. Phosphorylation of HnoC induces dissociation of the response regulator tetramer and detachment of subunits from the promoter DNA, which subsequently leads to transcriptional derepression.
Keywords: MerR; feedback; transcription factor; two-component signaling.