YSCM1 AND YSCM2, 2 YERSINIA-ENTEROCOLITICA PROTEINS CAUSING DOWN-REGULATION OF YOP TRANSCRIPTION

Synthesis of the Yop proteins by yersiniae is downregulated when secre tion is prevented by closure or destruction of the contact (type III) secretion channel. In Yersinia pseudotuberculosis, a mutation in the I crQ gene, encoding a secreted protein, reduces this feedback inhibitio n mechanism. Surprisingly, mutation in the yscM gene, the IcrQ homolog ue in Y. enterocolitica, does not lead to the same deregulated phenoty pe. In this paper, we addressed the question of this discrepancy. We f ound a new gene on the Y. enterocolitica pYV plasmid that encodes a pr otein with 57% identity to YscM (now called YscM1). Overexpression of this gene, called yscM2, like overexpression of IcrQ and yscM1, blocke d Yop secretion. A double yscM1, yscM2 mutant had the same phenotype a s that of the IcrQ mutant. The discrepancy can thus be explained by th e existence of two functionally equivalent copies of yscMin Y. enteroc olitica. 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, indicati ng 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 obstr ucting the secretion channel.