Conditional Mutator Gene in Escherichia coli : Isolation, Mapping, and Effector Studies

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Abstract
A mutator gene, mutD5 , whose phenotype is conditional, has been identified in Escherichia coli . By P1 transduction it has been shown to lie at about 5.7 min on the chromosome, being co-transduced with proA and argF . In rich medium, streptomycin- and nalidixic acid-resistant mutation frequencies are 50 to 100 times higher than those in minimal medium. In minimal medium, the mutD5 -induced mutation frequencies are still 50 to 100 times above co-isogenic wild-type ( mut + ) levels. Similar results were obtained with all markers tested. Mutant frequencies can be raised by thymidine in the medium at concentrations as low as 0.04 μM, or by the endogenous generation of thymidine from thymine plus a deoxyribosyl donor. Deoxyadenosine, various ribonucleosides, thymine, and 2-deoxyribose do not stimulate mutation. None of these effects are related to growth rate, since growth rate and mutation rate can be decoupled completely.