SECTION 5 



MUTAGENESIS 



5.1. Application of Radiation Denaturation of DNA 

 in Combination with Chemical Mutagens to 

 Affect the Mutational Process. R. I. Salganik 

 (Novosibirsk, U.S.S.R.). 



It has been established that some mutagens 

 (hydroxylamine, hydrazine, dimethylsulfate and 

 others) react much more intensively with heat 

 denaturated DNA than with the native one; but 

 applying these mutagens alone, excepting heat 

 denaturation of DNA, under the conditions of 

 our experiment, does not result in significant 

 breaking of hydrogen bonds in DNA. The 

 hydrogen bonds in double-strand structure of 

 DNA can also be broken by means of radiation 

 energy (ultraviolet radiation). This effect can 

 be realized not only in vitro but in vivo as well 

 and besides that it can be dosed. The raising of 

 the dose of the ultraviolet radiation increases 

 the number of denaturated regions in DNA. It 

 is reasonable to suppose that those regions of 

 DNA in which are prevailing weaker A-T bonds 

 and which are less saturated with stronger 

 G-C bonds are broken in the first place. 



It has been shown that radiation denaturation 

 also increases the ability of DNA to react with 

 investigated chemical mutagens. 



The selectivity of biological effect of applied 

 mutagens may be raised in connection with 

 their ability to react in denaturated regions of 

 DNA with certain nitrous bases (hydroxylamine 

 -> cytosine, hydrazine -> thymine, dime- 

 thylsulfate -> guanine). 



Stepped irradiation with denaturating doses 

 of ultraviolet rays in combination with certain 

 chemical mutagens has been applied for affecting 

 the frequency and the spectrum of mutations in 

 bacteria. 



mutagenic alkylating agent methyl methane- 

 sulfonate (MMS). The first is a local "de- 

 naturation" which occurs upon treatment of 

 DNA with either MMS, ultraviolet light, X-rays, 

 or when DNA is denatured by heat and then 

 annealed. This local reaction sensitizes the DNA 

 to inactivation by an enzyme found in B. 

 subtilis and in Micrococcus lysodeikticus. DNA 

 inactivation is apparently due to strand breakage. 



The second reaction results in alkylation of 

 the DNA. DNA carrying alkyl group(s) is active 

 for transformation and can replicate in vivo. 

 Alkylated DNA is exponentially inactivated by 

 heating well below the thermal denaturation 

 temperature. This inactivation is probably due 

 to loss of alkylated purines since enzymatic, 

 ultracentrifuge and thermal denaturation studies 

 indicate that alkylated and heated (50°C) DNA 

 need not contain single strand breaks. Apurinic 

 DNA is, therefore, inactive for transformation 

 and (we presume) for replication in vivo. 



Mutation induced by alkylating agents must 

 be due to mispairing while the alkyl group is 

 still part of the DNA molecule rather than at the 

 apurinic stage since apurinic DNA does not 

 replicate. This hypothesis is supported by the 

 results of studies on the reversion specificity of 

 lac~{rz~) strains of Escherichia coli obtained 

 by one of us (N.S.); lac~ mutations induced by 

 the alkylating agent ethyl methanesulfonate 

 behave as though they were transitions rather 

 than the mixture of transitions and transversions 

 expected if mutation occurred at the stage of 

 apurinic DNA. 



This work was supported by grants from the 

 National Institutes of Health (U.S.A.) and from 

 the National Science Foundation. 



5.2. Chemical and Biological Effects of the Reaction 

 of Deoxyribonucleic Acid with Mutagenic 

 Alkylating Agents. B. S. Strauss, R. Wahl, 

 M. Maillis, J. Coston and N. Schwartz 

 (Chicago, U.S.A.). 



Transforming deoxyribonucleic acid (DNA) 

 obtained from Bacillus subtilis undergoes at 

 least two reactions when treated with the 



5.3. Chemical Mutagenesis on Separated DNA. 



S. E. Bresler (U.S.S.R.). 



1. By means of bacterial transformation we 

 are able to make a quantitative study of chemical 

 mutagenesis on separated and purified DNA. 



To know the conditions of quantitative assay 

 for transforming activity of DNA the homogeneity 



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