Instability of the Salmonella RcsCDB signalling system in the absence of the attenuator IgaA.

IgaA is a Salmonella enterica membrane protein that attenuates the response of the RcsCDB signalling system to envelope stress. This protein is essential unless the RcsCDB system is inactivated, suggesting that IgaA may constantly adjust the magnitude of the response. Such a functional link is also supported by the concurrence of the igaA and rcsD-rcsB-rcsC loci in genomes of enteric bacteria and the selection of spontaneous mutations in the RcsCDB system following IgaA deprivation. However, the exact nature of the spontaneous mutations rendering IgaA dispensable remains undefined. In this work, we examined how the transduction of an igaA null allele affects the status of the RcsCDB system. Loss of RcsCDB response was registered in approximately 90 % of the IgaA-defective clones, which failed to produce the capsule material positively regulated by this system. About half of these non-mucoid clones suppressed the loss of IgaA with large deletions encompassing variable regions of the rcsD-rcsB-rcsC locus. Unexpectedly, mucoid transductants were also reproducibly obtained and indicated the capacity of S. enterica to retain a functional RcsCDB system in the absence of IgaA. Decreased levels of either RcsC or RcsD were shown in 'mucoid' clones lacking IgaA and displaying low responsiveness to stimuli. Taken together, these data demonstrate that the stability and responsiveness of the RcsCDB system relies on its attenuator IgaA. The type of suppressions found also support a model with IgaA controlling the level of signal flowing through RcsC and RcsD.