ANALYSIS OF THE BOUNDARIES OF SALMONELLA PATHOGENICITY ISLAND-2 AND THE CORRESPONDING CHROMOSOMAL REGION OF ESCHERICHIA-COLI K-12

We recently identified a pathogenicity island (SPI2) located at 30.7 c entisomes on the Salmonella typhimurium chromosome. SPI2 contains gene s encoding a type III secretion system whose function is distinct from that of the type III secretion system encoded by a pathogenicity isla nd (SPI1) at 63 centisomes which is involved in epithelial cell entry. An analysis of the boundaries of SPI2 and comparison with the corresp onding region of the Escherichia coli chromosome re,vealed that SPI2 i nserted adjacent to the tRNA(Val) gene. The E. coli chromosome contain s 9 kb of DNA at the region corresponding to the SPI2 insertion point which appears to be absent in S. typhimurium. The distribution of SPI1 and SPI2 was examined in various Salmonella isolates. In contrast to type III secretion system genes of SPI1, those of SPI2 are not present in Salmonella bongori, which diverged at the first branch point in th e Salmonella lineage, These and other data indicate that SPI2 was acqu ired by a Salmonella strain already harboring SPI1 by horizontal trans fer from an unknown source.