Yersinia enterocolitica type III secretion:: an mRNA signal that couples translation and secretion of YopQ

Pathogenic Yersinia species export Yop proteins via a type III machinery to escape their phagocytic killing during animal infections. Here, we reveal the type III export mechanism of YopQ. In the presence of calcium, when type III secretion was blocked, yopQ mRNA was not translated. The signal of YopQ sufficient for the secretion of translationally fused reporter proteins was contained within the first 10 codons of its open reading frame. Some frameshift mutations that completely altered the peptide sequence specified by this signal did not impair secretion of the reporter protein. Exchanging the upstream untranslated mRNA leader of yopQ for that of E. coli lacZ also did not affect secretion. However, removal of the first 15 codons abolished YopQ export. Pulse-labelled YopE, but not YopQ, could be secreted after the polypeptide had been synthesized within the cytoplasm of Yersinia (post-translational secretion). Thus, YopQ appears to be exported by a mechanism that couples yopQ mRNA translation with the type III secretion of the encoded polypeptide.