MUTAGENESIS 131 



I have been wondering whether any of the cases of mutagen stability 

 can be due to such an effect of the plating medium. Take for instance 

 Glover's (30) interesting data on E. coli. He treated a tryptophanless 

 strain with a variety of mutagens and scored reversions. In a certain 

 experiment, he obtained 249 reversions with UV, 55 with X-rays, and 

 none with diepoxybutane. So it seemed that this tryptophan allele was 

 specifically mutagen stable to diepoxybutane. But when he incor- 

 porated an arginine requirement in the same strain, the tryptophan 

 locus responded to the chemical. Witkin has recently reported a very 

 similar case. I feel that in these cases one should at least see whether 

 the plating medium is responsible, whether, for instance, in Glover's 

 case the apparent mutagen stability of the tryptophan locus in the 

 wild-type strain would disappear if one plated on arginine instead of 

 minimal medium. I don't say it would, but it would be worth testing. 



In addition, one must of course keep in mind that the whole bio- 

 chemical pattern in a cell is altered by introduction of a second mutant 

 gene, and that this alteration may affect various steps in mutagenesis 

 of the first gene. Clarke found that in fission yeasts both plating 

 medium and residual genotype influenced mutation frequency at the 

 ad-1 locus, for while adenine reversions in the single adenineless strain 

 were almost completely suppressed by methionine in the plating 

 medium, suppression was less complete in the double strain with 

 methionineless. It seems probable that the influence of the genotypic 

 background, too, may depend to some extent on the mutagen used, and 

 this is another possible explanation of the type of data Glover ob- 

 tained. In this case, too, mutagen specificity would not be likely to 

 reside in the reaction between mutagen and gene, for it is difficult to 

 imagine that this reaction should be controlled by other genes in the 

 same nucleus. 



I would like to discuss a few more possible ways by which mutagen 

 specificity may arise. One point that came out in the discussion this 

 morning is that some and perhaps all alkylating agents go on acting 

 for quite a long time after they have been washed out. This is so for 

 diepoxybutane and TEM (tri-ethylene melamine), and Freese men- 

 tioned that it is so for ethylethane sulfonate. If there should be a dif- 

 ference in this delayed effect on two loci, then any initial difference 

 in mutation frequency will be magnified the longer the material is kept 

 between treatment and plating. 



For instance, in the system used by Dr. K0lmark, which is an 

 adenine plus inositol requiring strain of Neurospora, diepoxybutane 

 acts preferentially on the adenine locus and very little on the inositol 



