YscM1 and YscM2, two Yersinia enterocolitica proteins causing downregulation of yop transcription

Synthesis of the Yop proteins by yersiniae is downregulated when secretion is prevented by closure or destruction of the contact (type III) secretion channel. In Yersinia pseudotuberculosis, a mutation in the IcrQ gene, encoding a secreted protein, reduces this feedback inhibition mechanism. Surprisingly, mutation in the yscM gene, the IcrQ homologue in Y. enterocolitica, does not lead to the same deregulated phenotype. In this paper, we addressed the question of this discrepancy. We found a new gene on the Y. enterocolitica pYV plasmid that encodes a protein with 57% identity to YscM (now called YscM1). Overexpression of this gene, called yscM2, like overexpression of IcrQ and yscM1, blocked Yop secretion. A double yscM1, yscM2 mutant had the same phenotype as that of the IcrQ mutant. The discrepancy can thus be explained by the existence of two functionally equivalent copies of yscM in Y. enterocolitica. Overexpression of yscM1 drastically reduced the expression of a yopH-cat reporter gene when tested in a pYV+ background. However, no effect could be observed in the absence of a pYV plasmid, indicating that YscM1 does not act directly as a transcriptional repressor or as an anti-VirF factor. We have also ruled out that YscM acts by obstructing the secretion channel.