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A spontaneous Escherichia coli K12 mutant which inhibits the excision-reintegration process of Mu gem2ts.

Escherichia coli K12 strains lysogenic for Mu gem2ts with the prophage inserted in a target gene (i.e., lacZ::Mu gem2ts lysogenic strains) revert to Lac+ by prophage precise excision with a relatively high frequency (about 1 x 10(-6)). The revertants obtained are still lysogens with the prophage inserted elsewhere in the bacterial chromosome. We have observed that, with the time of storage in stabs, bacterial cultures lysogenic for Mu gem2ts lose the ability to excise the prophage. The mutation responsible for this effect was co-transducible with the gyrB gene. After the removal of the prophage by P1 vir transduction from these strains, one randomly chosen clone, R3538, was further analyzed. It shows an increment of DNA supercoiling of plasmid pAT153, used as a reporter, and a reduced beta-galactosidase activity. On the other hand, R3538 is totally permissive to both lytic and lysogenic cycles of bacteriophage Mu.