GENETIC IMPLICATIONS OF MUTATIONS IN S. TYPHIMURIUM 



271 



were auxotrophic mutants, among which 17 different auxotroj^hic mutants 

 occur according to tests of the specific nutrilite required. A summary of these 

 requirements with the numbers of each is given in Table 17.2. Tlae most 

 frequent auxotroph is the one requiring cysteine. The next most frequent is 

 the histidine auxotroph, and so on down the list to one which has a double 

 requirement of both valine and isoleucine for growth. Only two auxotrophic 

 mutants require substances other than amino acids. One must be supplied 



TABLE 17.1 



FREQUENCIES OF AUXOTROPHIC MUTATIONS IN S. TYPHIMURIUM 

 AFTER X-RADIATION AND PENICILLIN SCREENING 



with adenine, and others (not found in this experiment) must have either 

 guanine or thiamin in the medium. 



In our published report of these data (Plough, Young, and Grimm, 1950, 

 Table 3) we listed a number of additional strains showing alternative re- 

 quirements. Davis (personal communication) retested a number of these 

 and found them to be mixtures of single autotrophs. We have just completed 

 an extensive recheck of all strains listed originally as alternates, and now 

 confirm his results except for the three types of alternates listed in Table 

 17.2 (Plough, Miller, and Berry, 1951). 



MUTATION FREQUENCY AND X-RAY DOSAGE 



One of the most interesting results of this experiment is the clear relation 

 between the frequency of auxotrophic mutants and the X-ray dosage. This 

 is shown in Table 17.1, column 5, lines II- VIII, and I could add to the data 

 from other experiments. The numbers of tests vary for the different radiation 



