SUMMARY: The isolation of chromosomal mutants that increased, the level of resistance of a partially tetracycline-sensitive mutant of R100-1 is described. Plasmid-less derivatives of these moderately resistant mutants were phenotypically similar to the mutants described by Reeve (1966, 1968), and also mapped in the same region. The level of intrinsic resistance to both chloramphenicol and tetracycline was increased about twofold. Also, the levels of R factor-determined resistance to these drugs were increased by this host mutation and tetracycline resistance was expressed constitutively. A mutant accumulated tetracycline at a threefold] lower rate than the wild-type strain, and it is proposed that the mutants have an altered permeability to the drugs and that this acts syner-gistically with the products of the R factor chloramphenicol and tetracycline resistance genes.


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