Molecular cloning, nucleotide sequence, and function of a site-specific recombinase encoded in the major `pathogenicity island' of Salmonella typhi

Abstract

The genome of the typhoid fever bacterium, Salmonella typhi, contains at least three large insertions (`pathogenicity islands') relative to the chromosome of Salmonella typhimurium (which is normally non-invasive for humans) [Liu, S.-L., Sanderson, K.E., 1995. Rearrangements in the genome of the bacterium Salmonella typhi. Proc. Natl. Acad. Sci. USA 92, 1018–1022]. DNA encoding a site-specific recombinase (the rci gene) and an adjacent putative pilus-tip adhesin protein (the pilV gene) was located (near min 94) in the major `pathogenicity island' of the S. typhi chromosome, cloned, and sequenced. It was shown that the Rci protein inverted a DNA segment of 490 bp, between two 19-bp inverted repeat elements, to place either of two possible C-termini on a constant N-terminal region of the PilV protein. Both possible PilV proteins were seen when the alternative pilV genes were transcribed from the T7 promoter and translated in vivo. Both the rci and the N-terminal region of the pilV gene show a high degree of homology to genes encoded by the IncI2 plasmid R721 and the IncI1 plasmid R64. One of the possible pilV C-termini (in the pilV1 gene) is highly homologous to shufflon C (one of the possible PilV C-termini) of R64; the other possible pilV C-terminus (in the pilV2 gene) shows no homology to any published shufflon. In the R64 plasmid, the PilV proteins are pilus-tip adhesins; different PilV proteins recognize different potential recipient bacterial strains as a prelude to mating in liquid culture [Komano, T., Kim, S.-R., Yoshida, T., Nisioka, T., 1994. DNA rearrangement of the shufflon determines recipient specificity in liquid mating of IncI1 plasmid R64. J. Mol. Biol. 243, 6–9]. It is likely that S. typhi encodes pili bearing two alternative PilV proteins as tip adhesins, one of which recognizes, specifically, a membrane component of Escherichia coli K-12, while the specificity of the other PilV protein is not known.