NssR, a member of the Crp‐Fnr superfamily from Campylobacter jejuni, regulates a nitrosative stress‐responsive regulon that includes both a single‐domain and a truncated haemoglobin

Consistent with its role as a nitric oxide (NO)‐detoxifying globin in Campylobacter jejuni, Cgb (Campylobacter globin) expression is strongly and specifically induced following exposure to nitrosative stress, suggesting a previously unrecognized capacity for NO‐related stress sensing in this food‐borne pathogen. In this study, Fur and PerR have been eliminated as major regulators of cgb, and NssR (Cj0466), a member of the Crp‐Fnr superfamily, has been identified as the major positive regulatory factor that controls nitrosative stress‐responsive expression of this gene. Accordingly, disruption of nssR resulted in the abolition of inducible cgb expression, which was restored by a complementing chromosomal insertion of the wild‐type gene with its indigenous promoter at a second location. The NssR‐deficient mutant was more sensitive to NO‐related stress than a cgb mutant and this phenotype most likely arises from the failure of these cells to induce other NO‐responsive components in addition to Cgb. Indeed, analysis of global gene expression, by microarray and confirmatory real‐time polymerase chain reaction (PCR) in the wild type and nssR mutant, not only confirmed the dependence of inducible cgb expression on NssR, but also revealed for the first time a novel NssR‐dependent nitrosative stress‐responsive regulon. This regulon of at least four genes includes Cj0465c, a truncated globin. Consistent with NssR being a Crp‐Fnr superfamily member, an Fnr‐like binding sequence (TTAAC‐N4‐GTTAA) was found upstream of each gene at locations −40.5 to −42.5 relative to the centre of the binding sites and the transcription start point. Site‐directed mutagenesis confirmed that this cis‐acting motif mediates the nitrosative stress‐inducible expression of cgb.