424 MUTATION AND PLANT BREEDING 



tion of incorporated phosphor us-3 2. Nitrous-acid-induced ill mu- 

 tants in T4 phage are, for the most part (87 per cent), reverted by base 

 analogues (25). 



In contrast to the sound theoretical basis (actually as yet unproved 

 by experiment) for the mutagenicity, and by plausible extension the 

 mutagenic specificity, of base analogues and nitrous acid, there is little 

 known at present which would explain mutagenic specificity of alky- 

 lating agents. These compounds react with proteins, nucleic acids, 

 and nucleoproteins; chiefly with sulphhydryl (— SH), amino (— NH 2 ), 

 and acid (-COOH) groups (54, 61). Reactions with the phosphate 

 groups, amino and imino groups, and ring nitrogens of the purines 

 and pyrimidines of DNA or precursors are, in all probability, of 

 importance in mutagenicity. It is not known which of these reactions 

 is primarily responsible for the ultimate genetic changes. Based on 

 in vitro reactions some investigators have been inclined to emphasize 

 esterification of the phosphate groups (1, 2), which results in the 

 formation of unstable triesters; others, the direct alkylation of the 

 purine or pyrimidine ring (40, 41). Ethylation in phage DNA may 

 be particularly significant and may lead to faulty replication (43). 

 Reiner and Zamenhof (53) reported differential sensitivity of DNA 

 bases of calf thymus when treated in vitro with the alkylating agents 

 dimethyl sulphate, diethyl sulphate, and nitrogen mustard. These 

 agents attach their alkyl group to the purines, but not the pyrimi- 

 dines, of DNA on the nitrogen in position 7 (72). 



The alkylated DNA cannot itself be the mutant hereditary deter- 

 minant since such a structure could not be replicated by metabolic 

 cell processes. A preferred interpretation is that alkylation of a DXA 

 group interferes with synthesis so as to increase the chance of errors 

 in copying (44). 



Szybalski (66) treated Escherichia coli cells with the alkylating 

 agent triethylenemelamine (TEM) and investigated the mechanism 

 by which mutants for streptomycin independence are produced. 

 TEM reacted extensively with pyrimidines and with their nucleo- 

 sides and deoxynucleosides, but not with the purines adenine and 

 guanine. Szybalski postulated several sequential reaction steps to 

 explain the effect of TEM: (a) semireversible absorption of TEM by 

 resting cells; (b) chemical reaction between the mutagen and pyrimi- 

 dine deoxynucleotides with the production of fraudulent DNA pre- 



