The SPI-3 pathogenicity island of Salmonella enterica

Pathogenicity islands are chromosomal clusters of pathogen-specific virulen ce genes often found at tRNA loci. We have determined the molecular genetic structure of SPI-3, a 17-kb pathogenicity island located at the selC tRNA locus of Salmonella enterica serovar Typhimurium. The G+C content of SPI-3 (47.5%) differs from that of the Salmonella genome (52%), consistent with t he notion that these sequences have been horizontally acquired. SPI-3 harbo rs 10 open reading frames organized in six transcriptional units, which inc lude the previously described mgtCB operon encoding the macrophage survival protein MgtC and the Mg2+ transporter MgtB. Among the newly identified ope n reading frames, one exhibits sequence similarity to the ToxR regulatory p rotein of Vibrio cholerae and one is similar to the AIDA-I adhesin of enter opathogenic Escherichia coli, The distribution of SPI-3 sequences varies am ong the salmonellae: the right end of the island, which harbors the virulen ce gene mgtC, is present in all eight subspecies of Salmonella; however, a four-gene cluster at the center of SPI-3 is found in only some of the subsp ecies and is bracketed by remnants of insertion sequences, suggesting a mul tistep process in the evolution of SPI-3 sequences.