pE194 is a multicopy, temperature-sensitive, 3.5 kb plasmid which confers resistance to erythromycin. Selection at nonpermissive temperatures for replication in the presence of antibiotic yields clones in which pE194, seemingly intact, has been integrated into the Bacillus subtilis chromosome. Integration is recE-independent, can occur at many sites on the chromosome, and sometimes results in the inactivation of chromosomal gene expression, presumably due to integration within a gene. We have mapped a preferred site within the pE194 genome, at which integration occurs. Two minor sites have also been mapped. Excision of pE194 from the chromosome occurs. This excision can probably be precise with regard to both the chromosome and the pE194 DNA, since it can be accompanied by restoration of function of an inactivated gene and by regeneration of seemingly intact autonomous plasmid. Imprecise excision also occurs, as does translocation from one chromosomal site to another. The frequency of integration and of excision resulting in restoration of chromosomal gene expression, are both about 10-8 to 10-9, and integration is not induced by growth at nonpermissive temperatures. The nature of these phenomena and their relationship to other illegitimate recombination events in B. subtilis and Staphylococcus aureus are discussed.
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