Signal-dependent requirement for the co-activator protein RcsA in transcription of the RcsB-regulated ugd gene

The RcsC/ YojN/ RcsB phosphorelay system controls gene expression in response to a variety of signals, including changes in temperature, osmolarity, and overproduction of membrane proteins. Transcription of certain RcsB- activated genes, such as the capsule synthesis cps operon, requires the co- activator protein RcsA, whereas expression of other RcsB- activated genes is RcsA- independent. We have established previously that a tolB mutation induces transcription of the Salmonella UDP- glucose dehydrogenase ugd gene in an RcsA- and RcsB- dependent manner. This induction is independent of the two- component systems PhoP/ PhoQ and PmrA/ PmrB, which are required for ugd expression in response to low Mg2+. We now report that the RcsC/ YojN/ RcsB system is activated in a pmrA mutant experiencing Fe3+ and low Mg2+, resulting in expression of both cps and ugd genes. However, whereas cps transcription remained RcsA- dependent, ugd transcription became RcsA- independent but dependent on the PhoP protein. S1 mapping experiments demonstrated that RcsA- dependent and - independent transcription of the ugd gene use the same promoter. DNase footprinting analysis identified a PhoP- binding site in the ugd promoter. Yet, PhoP- mediated ugd transcription required either the RcsC/ YojN/ RcsB or the PmrA/ PmrB systems.