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.