HOMOLOGOUS REGIONS OF THE SALMONELLA-ENTERITIDIS VIRULENCE PLASMID AND THE CHROMOSOME OF SALMONELLA-TYPHI ENCODE THIOL, DISULFIDE OXIDOREDUCTASES BELONGING TO THE DSBA THIOREDOXIN FAMILY

The nucleotide sequence relatedness between the chromosome of Salmonel la typhi and the virulence plasmid of Salmonella enteritidis was inves tigated using short DNA probes of < 2 kb covering the whole virulence plasmid sequence. Only one homologous region was detected. This region was subsequently cloned and partially sequenced. Sequences closely re lated to the pefI gene and the ORFs orf7, orf8 and orf9, which are loc ated downstream of the fimbrial pef operon of the Salmonella typhimuri um virulence plasmid, were detected. Sequencing of the cloned S. typhi DNA fragment also revealed identity with genes of the fimbrial sef op eron characterized in the chromosome of S. enteritidis. These nucleoti de sequences mapped upstream of the S. typhi chromosomal region homolo gous to the S. enteritidis virulence plasmid. The general organization of the cloned S. typhi chromosomal fragment was similar to the fimbri ae-encoding region of the S. typhimurium virulence plasmid. The deduce d product of orf8 in the S. typhimurium virulence plasmid, as well as those of the corresponding ORFs in the homologous region of the S. typ hi chromosome and in the S. enteritidis virulence plasmid (designated dlt and dip, respectively), appeared to be related to the thioredoxin family of thiol:disulphide oxidoreductases. The dip gene was able to c omplement the DTT-sensitive phenotype, the inability to metabolize glu cose l-phosphate and the low alkaline phosphatase activity of a dsbA m utant of Escherichia coli. The dlt gene partially complemented the lac k of alkaline phosphatase activity, but not the other mutant phenotype s. The products of both genes could be detected using the T7 RNA polym erase promoter expression system. The estimated molecular masses of th e products of the dlt and dip genes by SDS-PAGE were 26 and 23 kDa, re spectively, the first being in agreement with the deduced amino acid s equence and the latter, somewhat smaller. The processing of a possible leader peptide in the Dip protein, but not in the Dlt protein, could be responsible for this difference. The Dip protein appeared as a doub let band on SDS-PAGE, which is characteristic of the oxidized and redu ced states of this kind of protein.