Comparative genetics of the inv-spa invasion gene complex of Salmonella enterica

The chromosomal region containing the Salmonella enterica pathogenic island inv-spa was present in the last common ancestor of all the contemporary lineages of salmonellae. For multiple strains of S. enterica, representing all eight subspecies, nucleotide sequences were obtained for five genes of the inv-spa invasion complex, invH, invE, invA, spaM, and spaN, all of which encode proteins that are required for entry of the bacteria into cultured epithelial cells. The invE, invA, spaAM, and spaN genes were present in all eight subspecies of S. enterica, and for invE and invA and their products, levels of sequence variation among strains were within the ranges reported for housekeeping genes. In contrast, the InvH, SpaM, and SpaN proteins were unusually variable in amino acid sequence. Furthermore, invH was absent from the subspecies V isolates examined. The SpaM and SpaN proteins provide further evidence of a relationship (first detected by Li et al. [J. Li, H. Ochman, E. A. Groisman, E. F. Boyd, F. Solomon, It. Nelson, and R. K. Selander, Proc Natl. Acad. Sci. USA 92:7252-7256, 1995]) between the cellular location of the products of the inv-spa genes and evolutionary rate, as reflected in the level of polymorphism within S. enterica. Invasion proteins that are membrane bound or membrane associated are relatively conserved in amino acid sequence, whereas those that are exported to the extracellular environment are hypervariable, possibly reflecting the action of diversifying selection.