SUMMARY: Variants of a methicillin-resistant showing loss of or reduced resistance to the antibiotic were isolated at frequencies of 0·1--100% from cultures which had been starved, grown at elevated temperature, or given small doses of UV radiation. Three types of variant were identified on the basis of population distribution of resistance to the antibiotic, and field-inversion gel electrophoresis of digests of the chromosome cut with the rarecutting restriction endonuclease I. Type I variants are methicillin-sensitive and have a deletion in the region of the chromosome. Type II varinats show reduced methicillin resistance and rearranged DNA elsewhere in the chromosome. Type III variants show reduced methicillin resistance and no detectable change in the chromosome. Type I deletions were mapped using cloned fragments from the region. In 13 of the 16 independently isolated deletion mutants, one of the deletion endpoints appears to correlate with the positions of insertion sequences or transposons found in this region of the staphylococcal chromosome.


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