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Nonchromosomal Antibiotic Resistance in Bacteria V. Isolation and Characterization of R Factor Mutants Exhibiting Temperature-Sensitive Repression of Fertility

Several distinct types of derepressed R factor mutants have been isolated and identified following nitrosoguanidine mutagenesis. One class of mutants appears to synthesize a temperature-sensitive repressor substance, as shown by the effects of temperature on R factor transfer, on male-specific phage sensitivity, and on F factor and Hfr transfer. In addition, two classes of repressor-in-sensitive derepressed R factor mutants have been identified: although both inhibit F factor transfer at 42 and 32 C, one mutant (R6-5drd50) represses its own fertility at only the low temperature. These results imply the existance of a plasmid-specific component of the R factor repression system, in addition to a component which acts on both R and F factor fertility.