CHARACTERIZATION OF THE SHIGELLA-FLEXNERI IPGD AND IPGF GENES, WHICH ARE LOCATED IN THE PROXIMAL PART OF THE MXI LOCUS

The Shigella flexneri invasion process requires the synthesis of the I pa proteins and their secretion by specific factors encoded by the mxi and spa genes, which are clustered upstream from the ipa operon. We r eport here the characterization of the ipgD, ipgE, and ipgF genes, whi ch are located in the 5' end of the mxi locus. Analysis of IpgF-PhoA f usions endowed with high levels of alkaline phosphatase activity confi rmed the functionality of a classical signal sequence detected in the sequence of IpgF. The ipgD and ipgF genes were each inactivated on the large virulence plasmid by insertion of a nonpolar cassette; each of the ipgD and ipgF mutants thus constructed showed the same invasive ph enotype as the wild-type strain and was also able to provoke keratocon junctivitis in guinea pigs. It thus appears that two genes located at the ipa-proximal part of the mxi locus are not directly involved in in vasion. Analysis of concentrated culture supernatants of the wild-type and ipgD strains indicated that secretion of one polypeptide, whose s ize was consistent with that predicted for the IpgD protein (60 kDa), was abolished in the ipgD mutant.