Section 5 — Mutagenesis 



relative specificity of mutagens may exist not 

 only on the allele level, but also on the intergenic 

 one, and that different mutagens may act with 

 different intensity on individual whole loci. 



5.13. Comparison of Ultraviolet and Ethyl Methane 

 Sulphonate induced Mutations of Adenine 

 Loci in Saccharomyces cerevisiae. W. P. Cos- 

 tello, E. A. Bevan and M. W. Miller 

 (Oxford, Great Britain). 



Mutations at the adenine-2 locus in S. cere- 

 visiae cause the accumulation of a red pigment. 

 If an adenine-2 strain is treated with mutagenic 

 agents a small percentage of white colonies is 

 observed among the survivors. These are due to 

 either reversion to wild-type at the adenine-2 

 locus, or to a mutation at a locus prior to ade- 

 nine-2 in the pathway of adenine synthesis, which 

 prevents the accumulation of the red pigment. By 

 reference to their complementation reactions 

 with standard strains, these double mutants 

 can be classified into five adenine loci. 



Two of these loci, adenine-5 and adenine-7, as 

 designated by Roman, are very closely linked. 

 Our data suggest that the adenine-5/7 complex 

 may be interpreted as a single locus showing 

 complementing sub-units. 



A total of 27 u.v. and 69 EMS-induced mutants 

 of the general adenine-5-7 type were isolated and 

 classified further on their complementation inter- 

 reactions. Of the u.v.-induced mutants 4 were 

 ad-5, 21 ad-7, and 2 ad-5/7, the double mutant : of 

 the EMS-induced mutants 27 were ad-5, 39 ad-7, 

 and 3 ad-5/7, indicating a degree of specificity 

 for the ad-5 type mutation by EMS. 



Survival-mutation studies for buffered and 

 unbuffered EMS treatment indicate that while 

 the overall mutation rates of the two are compa- 

 rable with respect to survivors, EMS buffered 

 at pH 5.8 kills only 40 per cent over a period of 

 2 hr whereas unbuffered EMS in saline, in which 

 the pH falls from 7.0 to 2.8 over 2 hr, kills 80 per 

 cent. 



5.14. The Pattern of Some Biochemical Mutants of 

 E. coli after X-rays and Cancerogenic Sub- 

 stances. A. Rosener and H. A. Kunkel 

 (Hamburg, Germany). 



By means of methods published by Lederberg 

 and by Holliday it is possible to identify various 

 biochemical mutants of bacteria produced by 

 mutagenic agents and to analyse the distribution 

 of the different types. In our investigations 



on the radioresistant strain E. coli B/r most of 

 the spontaneous mutations to auxotrophy are 

 serine/glycine-dependent. Other biochemical 

 mutants occur just sporadically. Nearly the 

 same distribution is observed after X-irradiation. 

 Further experiments were cairied out with several 

 nitrogen-compounds. These substances which 

 belong to the group of nitrosamines or nitro- 

 samides were proved to be highly cancerogenic 

 by Druckrey et al. Furthermore they are organ- 

 otropic, too, and produce malignant tumors in 

 different organs according to their chemical 

 structure. After application of these agents 

 a specific pattern of biochemical mutants is 

 observed which differs from the "spectrum" 

 obtained by X-irradiation or spontaneously in a 

 typical way. These results are discussed and 

 compared with the findings of similar experi- 

 ments which were carried out on Drosophila 

 in our laboratory, simultaneously. 



5.15. Reversion Patterns among Genetic Sites in 

 Salmonella typhimurium on Treatment with 

 Chemical Mutagens. A. Eisenstark and 

 Ruth VanSickle (Manhattan, U.S.A.). 



Initially, auxotrophic mutants of Salmonella 

 typhimurium LT2 were obtained by induction of 

 prototrophic cells with ultraviolet, X-rays, 

 neutrons, nitrous acid (NA), 2-aminopurine 

 (AP) and diethylsulfate (DES). The collection 

 also included a number of spontaneous mutants. 

 Approximately 400 of the single-site auxotrophic 

 mutants were examined for reversion to proto- 

 trophy following treatment with a series of 

 chemical mutagens. Among the observations 

 were: (1) the method of induction of forward 

 mutation influenced the reversion pattern in a 

 distinct manner; (2) base analogues failed to 

 revert over half of the NA mutants, indicating 

 that NA was not restricted to transition capa- 

 bility; (3) N-Methyl-Ni-nitro-N-nitrosoguani- 

 dine reverted the same sites as DES, but the two 

 showed distinct quantitative differences for 

 certain specific sites; (4) 4,5,6 triaminopyrimi- 

 dine-sulfate-hydrate was highly mutagenic, but 

 reverted a smaller number of sites than did AP; 

 (5) deletions were not observed when either AP 

 or DES were used to induce mutation in the 

 prototrophic strain; (6) reversion patterns were 

 independent of either lysogeny or the presence 

 of a second mutation within the strain tested; 

 (7) a large fraction of apparent reversions did 

 not involve restoration of true wild-type, indi- 

 cating possible mutation at a suppressor locus. 

 These could be easily scored as stable small- 

 colony-formers when plated on minimal medium. 



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