Pathogenic yersiniae secrete a set of 11 anti-host proteins called Yops. The yop genes, scattered around the pYV plasmid, constitute a thermoinduced regulon controlled by the product of virF gene. The secretion of the Yops also requires the presence of the products of the other vir genes and operons, namely virA, virB and virC. The large virC operon and presumably some genes of the virA region encode a new secretion system. Mutations in any of these vir genes impair the production of all the Yops. In contrast, mutations in the yerA locus, located close to yopE, specifically abolish the expression of the cytotoxin YopE. We describe here the counterpart of yerA in Yersinia enterocolitica W22703. We demonstrate that the gene product of yerA regulates the production of YopE at a post-transcriptional level. It specifically binds the YopE protein. We consider that it acts as a specific chaperone and we call it SycE (for specific YopE chaperone). We hypothesize that SycE is a link between translation and the specific Yop export machinery. It is the first representative of a new family of pYV-encoded proteins.