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The plasmid RP1 was shown to contain a genetic region (the irp region) responsible for influencing the intrinsic resistance of Escherichia coli to penicillins but not to cephalosporins. Mutants in which the irp genes are inactive were isolated. RP1 carrying functional irp genes protected E. coli AS19 against lysozyme lysis and also enhanced resistance to actinomycin D, to nalidixic acid, and to rifampin. This plasmid also phenotypically repaired the hypersensitivity to penicillins of strain AS19, and also that of E. coli envA mutants. Similar regions were not detected on the plasmids R1 and R55.
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Curtis et al. (1974) studied this question.
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