The transcriptional regulator IscR integrates host-derived nitrosative stress and iron starvation in activation of the operon in .

For successful infection of their hosts, pathogenic bacteria recognize host-derived signals that induce the expression of virulence factors in a spatiotemporal manner. The fulminating food-borne pathogen produces a cytolysin/hemolysin protein encoded by the operon, which is a virulence factor preferentially expressed upon exposure to murine blood and macrophages. The Fe-S cluster containing transcriptional regulator IscR activates the operon in response to nitrosative stress and iron starvation, during which the cellular IscR protein level increases. Here, electrophoretic mobility shift and DNase I protection assays revealed that IscR directly binds downstream of the promoter P , which is unusual for a positive regulator. We found that in addition to IscR, the transcriptional regulator HlyU activates transcription by directly binding upstream of P , whereas the histone-like nucleoid-structuring protein (H-NS) represses by extensively binding to both downstream and upstream regions of its promoter. Of note, the binding sites of IscR and HlyU overlapped with those of H-NS. We further substantiated that IscR and HlyU outcompete H-NS for binding to the P regulatory region, resulting in the release of H-NS repression and induction. We conclude that concurrent antirepression by IscR and HlyU at regions both downstream and upstream of P provides with the means of integrating host-derived signal(s) such as nitrosative stress and iron starvation for precise regulation of transcription, thereby enabling successful host infection.