Stability of plasmids R1-19 and R100 in hyper-recombinant Escherichia coli strains and in Salmonella typhimurium strains

Molecular rearrangements between two plasmids of the FII incompatibility group in different recombination--deficient Escherichia coli strains

The frequencies and types of plasmid molecular rearrangements generated in different recombinant mutants which carried two plasmids of the FII incompatibility group were studied. The wild-type cells generated molecular rearrangements mainly by interplasmidic recombination with a frequency of 2.4 x 10(-6) per cell per cell doubling. Cells in which RecF was the principal recombination pathway generated different types of molecular rearrangements that involved either both plasmids or one of the plasmids and the chromosome. The frequencies of molecular rearrangements for these cells were 50-fold greater than those of wild-type cells. The recA- cells, even when the RecE pathway was derepressed, generated rearrangements only between one of the plasmids and the chromosome, at very low frequencies (10(-9]. In wild-type cells and in RecF cells, interplasmidic recombination generated mainly cointegrates carrying DNA deletions. These cointegrates were stable in recA- or recA- RecE+ cells, but unstable in wild-type or RecF+ cells. In the latter, the cointegrates generated smaller plasmids with different molecular structures at relatively low frequencies.

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.