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Thymine-requiring mutants of Escherichia coli were rarely found before the observation by Okada, Yanagisawa, and Ryan (Z. Vererbungslehre 92:403, 1961) that cultures of bacteria grown in the presence of high concentrations of aminopterin, thymine, purines, and serine contain a surprising number of cells unable to synthesize their own supply of thymine. They showed that these thymine-dependent mutants could grow much faster in the presence of aminopterin than the parent strains, even though both were supplied with adequate amounts of those metabolites whose syntheses are blocked by aminopterin. These observations provided the basis for a widely used method of isolation of this special class of mutants. The greater efficiency of a simplified medium was reported by Okada, Homma, and Sonohara (J. Bacteriol. 84:602, 1962). We also found that the yield of thymutants was greatly increased if a completely minimal medium, usually M9, containing only thymine (50 ,g/ml) in addition to aminopterin (-300 ,ug/ml) was used as the isolation medium.

A radiosensitive mutant E. coli B,-, was discovered by Hill. 1 2 It exhibits the same plating efficiency as the parental strain E. coli B for normal T1 bacteriophage, but a greatly reduced efficiency with ultraviolet (UV) irradiated T1 phage.A The sensitive mutant apparently lacks some factor capable of reactivating certain photoproducts in the UV-irradiated DNA of T1 phage. Sensitive mutants of E. coli K12 which show the same low efficiency for scoring UV-irradiated T1 phage were induced by nitrous acid treatment.4 This paper reports experiments on genetic crosses between a sensitive F-strain and several resistant Hfr strains of K12. The inheritance of resistance to UV-irradiation among the progeny from the crosses was analyzed following methods similar to those previously reported,5 and a locus on the male chromosome was identified that can transmit ITV resistance to the zygote. This locus will be referred to as UVR or UVs ac

Relative differences in radiosensitivity between Escherichia coli strain B and the mutant strains, B/r (more resistant than B) and two types of B, (more sensitive), are maintained with respect to inactivation by ultraviolet radiation, X-rays and decay of incorporated 32P. The differences cannot be ascribed to variations in number of nuclei or in content of DNA or RNA. The effectiveness of various postirradiation and post-decay treatments in preventing inactivation varies for each strain. With the exception of reactivation by light, strain B/r is less reactivable than B and B,.The strains do not differ in their rates of mutation to resistance to bacteriophage TI. However, the rates of mutation of the B, mutants to the first step of furacinresistance are considerably lower than the rate for the parent strain B. From one of the B, mutants, new strains were obtained whose properties are mixtures of those of B and B/r.

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