MUTAGENESIS 99 



13) ; the range of increase of mutation rates was from about ten times 

 to a hundred times, using various markers such as streptomycin de- 

 pendence, thymine dependence, streptomycin sensitivity, and so on. 

 Recently, this mutagencity was also found in phage by Stent's group. 



Our thinking was first in terms of the methyl group on the fifth 

 carbon of the uracil ring; since we found that substitution of this 

 methyl group by bromine produced mutations (87), we thought that 

 making this group slightly heavier by replacing hydrogen by deuterium 

 would also have an effect. 



I want to say right away that we have no proof whatsoever that 

 deuterium acts through DNA. It may affect other parts of the cell, 

 as Dr. Auerbach mentioned. Still, it is tempting to speculate on this 

 subject, and there is some evidence that if deuterium goes into the 

 molecule of thymine, it goes also to the methyl group. 



We analyzed such DNA and, indeed, found that in the thymine of 

 DNA from bacteria grown on heavy water, out of four nonlabile 

 hydrogens, two were replaced by deuterium. Then we took this 

 thymine, which had the deuterium, and, using a thymine requiring 

 strain, incorporated it again into DNA, but only as thymine, to see 

 whether this procedure was mutagenic. It was completely nonmuta- 

 genic. 



We investigated other bases, purines and cytosine, which had deu- 

 terium in the molecule, using strains which need this particular purine 

 or pyrimidine and incorporate it into DNA. It turns out that there, 

 too, the bases are not mutagenic. It seems that the only plausible 

 conclusion would be that either mutagenicity is through a change of 

 hydrogen bonds into deuterium bonds, or else mutagenicity of deu- 

 terium is not due to deuteration of DNA. 



Auerbach: We will now leave the purines and pyrimidines and re- 

 lated substances. I should like to say a few words about the action 

 of other mutagens, for which the relation to DNA is less obvious. 

 There are first the alkylating agents, which for a long time were the 

 best-known mutagens. They can be divided into chemical classes 

 according to the effective alkylating group they carry. The best known 

 of these are: the chloroethyl group which occurs in mustards; the 

 ethylene imine or ethylene oxide group; and the methane sulphonate 

 group. 



In higher organisms, there is no way of knowing whether these 

 substances act directly by alkylation of DNA or more indirectly. 

 One thing emerged from work on Drosophila. There it could be shown 

 that at least one of these substances, mustard gas, acts as "hit poison" 



