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The extraordinary ultraviolet (u.v.) radiation resistance of Micrococcus radiodurans is due to the highly effective dark-repair of pyrimidine dimers and involves both an excision (Boling & Setlow, 1966) and a recombination repair mechanism (Moseley, Mattingly & Copland, 1972). This high resistance to U.V. radiation, which is about 20 times greater than in repair-competent Escherichia coli (Setlow & Duggan, I 964), makes M. rudiodurans an especially attractive model for studying repair processes. The availability of thyminedependent mutants of E. coli and M . radiodurans makes it possible to study the influence . of thymine starvation on the course of post-irradiation repair processes. Incubation of E. coli B/r thy in a medium without thymine results in an exponential decrease in the number of viable bacteria (Barner & Cohen, 1954) and in changes in the sensitivity of bacteria to u.v.-irradiation (Smith, Hodgkins & O'Leary, 1966). More recently, it has been shown that pre-irradiation inhibition of DNA synthesis induced by treatment with cytidine (Brozmanovh, Magkovh & Sedliakovh, I 973) or by starvation for thymine (Sedliakovi, Magek & Brozmanovh, 1972) causes a reduction in the amount of dimer excision. The extent of this reduction depends on the duration of the pre-irradiation thymine starvation period (Balgav9, Magek & Turek, 1975). However, by simultaneous starvation for thymine and amino acid before u.v.-irradiation, a reduction in dimer excision can be induced in various excision-proficient strains of E. coli. The starvation need cause neither death of the bacteria during the treatment nor an increase in their sensitivity to u.v.-irradiation (Sedliakovii et al., 1974). The purpose of the present study was to find out whether dimer excision after thymine starvation is affected in a similar manner in M. radiodurans.

Normal Excision of Pyrimidine Dimers after Thymine Starvation in Micrococcus radiodurans