Frequency of IS1-mediated molecular events in different members of the family Enterobacteriaceae.

The numbers of chromosomal copies of the insertion sequence IS1 in strains of Salmonella typhimurium (0 to 8 copies), Shigella sonnei (56 copies), and Shigella flexneri (41 copies) isolated in Mexico City, Mexico, were similar to those reported for these genera isolated in other countries. Of the 11 Shigella strains studied, all carried several small plasmids; however, in only one of these strains did a small plasmid contain IS1, IS1 recombination, cointegrate formation mediated by IS1 or by the IS1-flanked transposon Tn9, and transposition of Tn9 occurred at a higher frequency in S. typhimurium than in either Escherichia coli or S. sonnei strains. The frequencies of IS1 recombination in S. typhimurium strains containing either zero or eight copies of IS1 were similar.