SUMMARY: Three temperature-sensitive mutants of have been isolated which, unlike the wild-type, are unable to synthesize DNA at 39°. Synthesis of DNA stops immediately the bacteria are raised to the restrictive temperature. The mutants can be transformed normally for single genetic markers and can regain wild-type temperature resistance on incubation with DNA from wild-type at a frequency associated with single or very closely linked markers. Each mutant can also be transformed to wild-type temperature resistance with DNA from the other temperature-sensitive mutants. All three mutants are resistant to the lethal action of ionizing and ultraviolet radiations and at 39° are able to carry out all or most of the DNA repair functions associated with excision repair, but at a reduced rate compared with wild-type. The rate of excision at 39° of u.v.-induced thymine-containing pyrimidine dimers is four to five times slower in the mutants than in the wild-type.


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