FUNCTIONAL-ANALYSIS OF SSAJ AND THE SSAK U OPERON, 13 GENES ENCODING COMPONENTS OF THE TYPE-III SECRETION APPARATUS OF SALMONELLA PATHOGENICITY ISLAND-2/

We have investigated the structure and transcriptional organization of 13 genes of Salmonella Pathogenicity Island 2 (SPl2) that encode comp onents of the second type III secretion apparatus of Salmonella typhim urium. ssaK, L, M, V, N, O, P, Q, R, S, T, U constitute one operon of 10kb. ssaJ lies upstream of ssaK and is the terminal gene of another o peron. The deduced products of ssaJ, ssaK, ssaV, ssaN, ssaO, ssaQ, ssa R, ssaS, ssaT, and ssaU show greatest similarity to the Yersinia spp. genes yscJ, yscL, IcrD, yscN, ysc4 yscQ, yscR, yscS, yscT, and yscU, r espectively. The products of the ssaL, ssaM and ssaP genes do not have significant similarity to products of other type III secretion system s, and might be important for the specific function of the SPl2 type I II secretion system. Bacterial strains carrying different ssa mutation s display minor alterations in terms of serum sensitivity when compare d with the wild-type strain, but none are defective in replication wit hin macrophage-like RAW 264.7 cells. However, some of the ssa mutant s trains invade HEp2 cells less efficiently and are less cytotoxic to RA W 264.7 macrophages than the wild-type strain. We show that the invasi on defect is correlated with a lack of SipC in culture supernatants of these mutant strains. SipC is a product of the SPl1 type III secretio n system of S. typhimurium, and is important for epithelial cell invas ion. Therefore, mutations in SPl2 can affect the SPl1 secretion system , which raises the possibility of an interaction between the two type III secretion systems.