LrhA regulates rpoS translation in response to the Rcs phosphorelay system in Escherichia coli

Regulation of the Escherichia coli stationary-phase sigma factor RpoS is complex and occurs at multiple levels in response to different environmental stresses. One protein that reduces RpoS levels is the transcription factor LrhA, a global regulator of flagellar synthesis. Here we clarify the mechanism of this repression and provide insight into the signaling pathways that feed into this regulation. We show that LrhA represses RpoS at the level of translation in a manner that is dependent on the small RNA (sRNA) chaperone Hfq. Although LrhA also represses the transcription of the sRNA RprA, its regulation of RpoS mainly occurs independently of RprA. To better understand the physiological signals affecting this pathway, a transposon mutagenesis screen was carried out to find factors affecting LrhA activity levels. The RcsCDB phosphorelay system, a cell envelope stress-sensing pathway, was found to repress lrhA synthesis. In addition, mutations in the gene encoding the DNA motor protein FtsK induce lrhA synthesis, which may explain why such strains fail to accumulate RpoS in stationary phase.