| Pseudomonas aeruginosa is a highly virulent, opportunistic pathogen that has the ability to cause both acute and chronic infections in susceptible hosts. The ability to cause these two distinct types of disease is dependent on the coordinate regulation of multiple virulence factors. We have identified a hybrid two component signaling protein, RetS (RtsM), which is required for the expression of one of these virulence factors, the type III secretion system, in response to both in vitro inducing conditions and host cells. In addition, a strain in which retS is deleted (DeltaretS) is markedly less virulent in a murine model of acute pneumonia, demonstrating that RetS is required in vivo.; RetS is an unusual signaling protein with two tandem response regulator receiver domains. An analysis of the predicted phosphoacceptor sites of RetS found that receiver domain 1 is not required for the function of RetS and phosphorylation of this domain may serve to inhibit RetS signaling. Conversely, the residues important for phosphorylation of the second receiver domain are critical for RetS function. In addition, our data suggests that this domain may serve as a substrate for an additional sensor kinase or may function in a phosphatase reaction. These findings, in conjunction with the work of others, support a model in which regulation by RetS is accomplished by affecting the GacS/GacA global regulatory pathway and changing the levels of a small regulatory RNA, rsmZ.; We predict that signaling through RetS to induce expression of the type III secretion system is activated by sensing a particular signal, although it has not been identified. Our work demonstrates that the periplasmic domain of RetS is not required to activate expression of the type III secretion system, but may function to repress RetS activity in vivo. Conversely, the transmembrane domains of RetS may play an active role in signal sensing. Although independent of RetS, our experiments provide evidence that particular amino acids and the growth rate of the bacteria may play a role in the induction of the type III secretion system. |
