Unstable Mutants of R Factor

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Thirty mutants sensitive to tetracycline were obtained from an R100 factor capable of conferring resistance to tetracycline (TC), chloramphenicol (CM), streptomycin (SM) and sulfanilamide (SA). Among the TC sensitive mutants, three showed a high frequency of spontaneous loss from host strains. The genetic loci governing the stability of R factor in host bacteria were denoted as stb. The stb- R factors have lost many of the properties of a wild type R factor, such as, the capability to sexually transfer drug resistance and host chromosome, to confer superinfection immunity and to inhibit F function. All of these properties did not revert to a wild type phenotype, suggesting that these mutations are deletions including genetic determinants governing both TC resistance and stability of R factor. Recombinational analysis between stb- and stb+ R factors indicated that crossovers between the stb loci and those governing CM (or SM.SA) resistance took place at high frequency. No crossovers were detected between stb loci and those governing TC resistance, indicating that the stb loci are linked closely to the loci governing TC resistance.

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