420 MUTATION AND PLANT BREEDING 



Table 1. — Induction of Reverse Mutations with Chemical Mutagens in 31 

 Mutagen-labile Alleles at the Histidine Locus of Salmonella typhimurium. * 



*After Kirchner (37). See text for meaning of symbols. 



mutational pattern. The interpreted significance of these results will 

 be discussed below under mechanisms of mutagenic action. 



Patterns of reversion of 22 mutants at two tryptophan loci (try C 

 and try D) in Salmonella typhimurium have been investigated follow- 

 ing treatment with four chemical mutagens (15, 56). The compounds 

 used were the base analogues 5— bromouracil (5BU) and 2— amino- 

 purine (2AP), and the alkylating agents diethyl sulphate (DES) and 

 beta-propiolactone (BPL). Many of the alleles responded differently 

 to the treatments, as shown in Table 2, where + indicates that rever- 

 sions were induced and — that there was no increase in frequency of 

 reversions. As with alleles at the histidine locus, those tested at the 

 tryptophan loci also showed marked specificity to chemical mutagens. 

 DES induced reversions in all 18 of the mutants that reverted, of 



Table 2. — Induction of Reverse Mutations with Chemical Mutagens in 22 Alleles 

 at Two Tryptophan Loci of Salmonella typhimurium. * 



Alkylating agents Base analogues 



DES BPL 2AP 5BU 



No. of alleles 



+ + + + 9 



+ + + 1 



+ + + 5 



+ + 2 



+ 1 



4 



•After Balbinder in Demerec, et al. (15). See text for meaning of symbols. 



