Factors determining frequency of plasmid cointegration mediated by insertion sequence IS 1

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Abstract
Mutants with deletions at either end of the insertion sequence IS1 lose the ability to mediate cointegration of 2 plasmids in Escherichia coli, whereas mutants with deletions or an insertion within IS1 can mediate cointegration at a reduced frequency. These results, together with the nucleotide sequence analysis of the IS1 mutants, indicate that the 2 ends of IS1 (insL and insR) and 2 genes (insA and insB) that are encoded by IS1 are required for cointegration. Using a plasmid carrying 2 copies of IS1 it was found that the individual IS1 species mediate cointegration at different characteristic frequencies, and that each of 2 parts of plasmid DNA segments flanked by the 2 IS1 species is a transposon, mediating plasmid cointegration at a unique frequency. When 1 IS1 was replaced with a mutant IS1, the remaining wild-type IS1 complemented the cointegration ability of the mutant IS1 as well as a resulting mutant transposon that was then flanked by a wild-type IS1 and a mutant IS1. The efficiency of this complementation reflected the characteristic ability of an individual IS1 present on the plasmid to promote cointegration. The IS1-encoded proteins are produced in different amounts, depending on the location of IS1 in the plasmid, and these amounts determine the efficiency of complementation of the cointegration ability of a mutant IS1 as well as a mutant transposon. The location of an individual IS1 itself can also determine the frequency of cointegration in the presence of a given amount of the IS1 proteins. On the basis of the observation that the cointegration ability of a mutant IS1 is less efficiently complemented than is the ability of a mutant transposon, it is also suggested that the IS1-encoded proteins can function in trans, but act preferentially on the IS1 or transposon sequence from which they are produced in promoting cointegration.