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SUMMARY: The isolation of Escherichia coli 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 cmlB 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 cmlB 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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