Section 5 — Mutagenesis 



to M + and that of direct mutations to S r per 

 10" colonies was estimated. According to the 

 third characteristic variations with respect to 

 the level of vitamin formation were determined. 



The results of the study showed that compound 

 No. 6 by its effect with respect to the frequency 

 of mutations in loci M and S is equal to embichin, 

 chlorambucyl, sarcolysin and ultraviolet light; 

 10-20 times superior to diethylsulfate with 

 respect to the mutation frequency and much 

 more superior to these mutagens by the action 

 on induced variation with respect to vitamin 

 formation. Compounds Nos. 31, 32 and 33 by 

 their effect on methionine deficiency and vitamin 

 formation are 5-6 times superior to all these 

 mutagens, including ultraviolet light. 



It should be noted that these compounds 

 have a different effect on the two loci. Compound 

 No. 32 increases the mutation frequency by 

 37.000 times with respect to methionine de- 

 ficiency as compared to the frequency of spon- 

 taneous mutations, thus being 70 times superior 

 to ultraviolet light by its mutagenic effect on 

 the same locus, whereas with respect to strepto- 

 mycin sensitivity this compound increases the 

 frequency of mutations only by 150 times as 

 compared to the frequency of spontaneous 

 mutations, thus being 7 times inferior to the 

 ultraviolet light with respect to the mutation 

 frequency induced by this mutagen. No such 

 markedly differentiated effect on the two loci was 

 observed in compounds Nos. 31 and 33. 



On the basis of these results it is possible to: 

 (1) discuss some problems of the mechanism of 

 chemical mutagenesis; (2) consider the possi- 

 bility of differentiated mutability of various 

 genes under the action of the same mutagenic 

 factor and (3) assume that new compounds with 

 a strong mutagenic and specific effect are found. 



5.11. Comparative Studies on the Mutagenic Action 

 of Carcinogenic Substances, Nitrous Acid and 

 Diethylsulfate in Saccharomyces cerevisiae. 

 F. K. Zimmermann, H. Marquardt and 

 R. Schwaier (Freiburg, Germany). 



Certain carcinogenic substances have been 

 described that are supposed to act by producing 

 diazoalkanes which alkylate cellular structures. 

 Since mutations can be induced by alkylation 

 also, we tried to compare the possible mutagenic 

 action of carcinogens to mutagens of known 

 mutational pathways: Nitrous acid as a transi- 

 tional and diethylsulfate as a transversional 

 agent. The carcinogenic substances were: 

 N-nitroso-N-ethylurethan (ethylating), N-nitro- 

 so-N-Methylurethan (methylating). 



The mutational activities were checked in a 

 backmutation test with four haploid, adenine 

 requiring strains of Saccharomyces cerevisiae 

 all mutated spontaneously at the ade-locus. 

 The mutagenic activities are compared on a 

 modified mutational/lethal hit base. 



All carcinogenic substances are potent mu- 

 tagens, the methylating compound being more 

 efficient than the ethylating one. There is a 

 striking difference between diethylsulfate and 

 N-nitroso-N-ethylurethane. In two strains only 

 the latter is active, in one strain diethysulfate 

 alone and in one both are active together. The 

 ethyl and the methyl nitroso compound, how- 

 ever, parallel each other qualitatively in all 

 strains. Excepting for one strain there is a fair 

 coincidence in activity with nitrous acid. 



Mutational differences between diethylsulfate 

 and N-nitroso-N-ethylure then may be due to dif- 

 ferences in chemical reactivity. But the similarity 

 of the nitroso compounds among each other and 

 to some extent to nitrous acid might indicate that 

 the mutagenic action resides in a mechanism 

 different from alkylation. 



5.12. Mutagenesis in Corynebacterium. J. Necasek 

 (Prague, Czechoslovakia). 



The possibility of demonstration of the relative 

 specificity of mutagens was studied by inducing 

 biochemical mutants in the strain Corynebac- 

 terium VUA 9366. This strain is biotin dependent, 

 produces large amounts of glutamic acid and 

 has a very low spontaneous and induced mu- 

 tation rate. Nitrogen mustard, ethyl methane- 

 sulphonate and ultraviolet light were used as 

 mutagens. Biochemical mutants were isolated 

 by the penicillin method. Mutants with identical 

 dependences isolated in a single experiment were 

 classifie das caused by a single mutation. On the 

 whole 25,000 isolates werein vestigated after ap- 

 plication of the three mutagens mentioned above. 

 Nearly 1 per cent nutritionally deficient mutants 

 was obtained, in which approximately one half 

 required specific amino acids for growth. 



The results indicate that there exists a quali- 

 tative difference between the mutagenic action 

 of alkylating agents and ultraviolet light. With 

 respect to arginine biosynthesis the alkylating 

 agents induce mutants requiring arginine (step 3) 

 and mutants requiring either arginine or citrulline 

 (step 2). On the other hand the ultraviolet light 

 induces mutants requiring also arginine and 

 mutants requiring either arginine or citrulline 

 or ornithine (step 1). The difference in frequencies 

 of mutations blocking step 1 and step 2 was 

 statistically significant. It is concluded that the 



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