592 RADIATION BIOLOGY 



was broken down into different degrees of dependence, as measured by 

 the minimum streptomycin requirement for each mutant, inasmuch as 

 the ultraviolet mutants showed the greatest and the 7 mutants the least 

 uniformity in this respect. Phage resistance, which under all circum- 

 stances arose much more frequently than streptomycin resistance of all 

 types taken together, showed less increase in frequency from either 7 or 

 ultraviolet radiation, relative to the spontaneous frequency, than did 

 streptomycin resistance. However, the 7 rate was four times as high, 

 relative to the ultraviolet rate, in the case of phage resistance, as it was 

 in the case of streptomycin resistance. Looking at the matter from 

 another angle, 7 rays resulted in rates of the two groups of mutations 

 which were four times as unequal as those produced by ultraviolet. 



As Newcombe points out, tests on crossable E. coli K12 (a strain not 

 used in his study) have shown that, in that strain at least, all the differ- 

 ent mutations to streptomycin resistance are alleles. It is therefore 

 probable that the differences in relative frequencies of different kinds of 

 streptomycin resistance found in Newcombe's work for the mutations 

 produced in different ways represent differences in the likelihood of one 

 or another kind of mutation occurring in a given gene, according to the 

 causative factor at work. 



In a different type of comparison of the spectra of mutations of E. coli 

 strain B/r, arising spontaneously and as a result of ultraviolet treatment, 

 Bryson and Davidson (1951) followed the pattern of mutations to resist- 

 ance to phages and of nutritional deficiencies, occurring in those cells 

 which became Tl-resistant mutants. It turned out that spontaneous 

 mutations giving resistance to phage Tl involved, in nearly 90 per cent 

 of cases, a simultaneously arisen change of one of the following kinds: 

 resistance to phage T5 (about 50 per cent of the Tl-resistant cases), a 

 tryptophane-requiring nutritional deficiency (about 30 per cent), or both 

 these characteristics at once (about 5 per cent), or resistance to all the 

 phages T3, T4, and T7, but not T5, along with the "tryptophaneless" 

 deficiency (about 3 per cent). In contrast to this, the mutations to Tl 

 resistance produced by ultraviolet were very seldom tryptophane defi- 

 cient, perhaps no oftener than would have been expected by the accidental 

 coincidence of two independent mutations to these traits. However, they 

 had T5 resistance associated with them still oftener (in some 92 per cent of 

 cases) than did the spontaneous mutations, yet no cases of resistance to 

 phages T3, T4, or T7 were found. It is true that they did occasionally 

 involve deficiencies for other metabolites, of types not found with the 

 spontaneous mutations to Tl resistance, but possibly these, like the few 

 cases of tryptophaneless among them, can be ascribed to an accidental 

 association, caused by the high rate of induced mutations. However that 

 may be, the striking difference in regard to association with tryptophane- 

 less must have been due to a difference between the nature of the mutation 

 process instigated by ultraviolet and of that occurring "spontaneously." 



