The regulation of the expression of type III secretion substrates in Yersinia enterocolitica

Several Gram-negative bacterial pathogens employ a common virulence strategy known as type III secretion to establish infections in animal and plant tissues. Yersinia enterocolitica is an intestinal human pathogen that uses the type III pathway to secrete virulence factors into the extracellular milieu and to inject effector proteins directly into the cytosol of macrophages upon entry to the host. This strategy of protein injection manipulates cellular pathways of the host, preventing phagocytosis of the pathogen and eventually results in apoptotic death of the macrophage, allowing for perpetuation of the bacterium in the lymphoid tissues. Y. enterocolitica secrete at least fourteen proteins called Yops (Yersinia outer proteins) via the type III pathway, and six of these proteins have a characterized effector function in eukaryotic cells. Expression of the effector proteins is tightly regulated and yersiniae respond to a distinct set of environmental cues to coordinate the expression/injection of effectors with host cell contact. YscM1 and YscM2 are type III secretion substrates that negatively regulate the expression of effector Yops. Relief of repression had been predicted to occur via secretion of YscM1 and YscM2 to the extracellular milieu, resulting in depletion of the factors from the bacterial cytoplasm. Here, it is demonstrated that YscM1 and YscM2 are injected into the cytosol of host cells, and that this process requires the specific Yop chaperone SycH, a factor required for the injection of the tyrosine phosphatase YopH. It is also demonstrated that YscM1 and YscM2 function to block the expression of effector yop genes at a post-transcriptional level, a process that also requires the factors YopD and LcrH (SycD). This post-transcriptional regulation targets a conserved sequence contained in the 5' untranslated region of yop mRNA's. Further, it is demonstrated that binding of YscM1 and YscM2 to SycH is sufficient for relief of the repression of effector Yop synthesis, independent of the type III injection step of YscM1 and YscM2. It is therefore proposed that YopD, LcrH, YscM1, and YscM2 coordinately block the translation of effector yop mRNA's until repression is relieved through binding of SycH to YscM1 and YscM2.