Nitric oxide sensing in Campylobacter jejuni by NssR

Pathogenic bacteria encounter the toxic free radical nitric oxide (NO) during infection via the activity of host nitric oxide synthases or via the reduction of nitrite and nitrate in the gut by commensal bacteria. Survival of bacterial pathogens during infection depends on various mechanisms for NO sensing and detoxification. The foodborne pathogen Campylobacter jejuni detoxifies NO to nitrate using a globin protein (Cgb), expression of which is induced strongly and specifically by the transcription factor NssR upon exposure to NO. Previous work indicates that nitric oxide does not directly modulate the interaction of NssR with target DNA, suggesting the existence of an indirect method for NO sensing. Bioinformatics analysis has revealed that NssR is likely to bind cyclic nucleotides, and cyclic-di-GMP (c-di-GMP) has previously been linked to NO sensing in other bacteria via a mechanism involving NO-responsive phosphodiesterase and cyclase enzymes. Structural modelling described herein is consistent with NssR binding to cyclic nucleotides, and tryptophan fluorescence quenching confirms the ability of NssR to bind to c-di-GMP, cAMP and cGMP. Subsequently, electrophoretic mobility shift assay (EMSA) suggests that that binding of c-di-GMP to NssR leads to increase in affinity for the cgb promoter. Finally, an in vivo reporter assay provides direct evidence that c-di-GMP can influence NssR activity. Taken together, these data are consistent with an in vivo mechanism whereby NO elevates c-di-GMP levels in the cell, which in turn activates NssR activity resulting in Cgb expression and NO detoxification. This work provides potential molecular explanation for how NO is sensed and detoxified in C. jejuni.