318 GENE MUTATIONS CAUSED BY RADIATION 



a kind of oxidation process involving the removal of hydrogen. And 

 though it seems unlikely, for the reasons given by Dickey, Cleland, and 

 Lotz (13), that the mutagenic change is an ordinary oxidizing process 

 (since cellular oxidations are such a commonplace and organic peroxides 

 are in most connections not very effective oxidants anyway), neverthe- 

 less it is now evident that oxygen-carrying radicals of certain kinds 

 can in some way effect mutagenesis and that some of them can also 

 eause alterations in sulfhydryl-containing proteins. At any rate, it is 

 found by Barron that extra sulfhydryl groups, supplied to a medium, 

 tend to protect proteins (enzymes) having organically bound sulfhydryl 

 groups from attack by radiation. This finding has been followed up by 

 the work of Patt et at. (64), showing that cysteine tends to protect 

 rodents from radiation sickness, even though Le May {Proc. Soc. Exptl. 

 Biol. Med., 77: 337-339, 1951) has more recently reported no inhibition 

 of — SH enzymes in vivo by x-radiation. The ciuestion thus arises, 

 how far is there a parallelism between the processes studied by Barron 

 and those involved in mutagenesis, and to what extent may they be 

 identical? May it not be, for example, as Barron (in a personal com- 

 munication) has suggested as one possibility, that sulfhydryl groups 

 of the genetic nucleoprotein itself are attacked when a mutation is 

 produced by radiation? 



As everyone recognizes, despite the provocative points of agreement 

 in the chemical work carried on by very diverse methods, we still are 

 far from having anything like a complete picture.* There are results 

 which indicate that the reaction chains leading to mutagenesis have 

 various links, and that in fact the chains may branch and/or anasto- 

 mose. Moreover, there may be a number of different chains, having 

 more or fewer features in common, but all capable of leading to the end 

 result in question — permanent change of the genetic material. 



A possible illustration of this complexity is afforded by the finding of 

 Stone and his coworkers that, although very short ultraviolet was ef- 

 fective in making the medium temporarily mutagenic, that of longer 

 wave length was incapable of doing so. Yet this longer ultraviolet was 

 still in the mutagenic range (shorter than 3200 A) : that is, if absorbed 

 directly by the organisms it did produce mutations. It would seem, 

 then, as if the shorter-range effect that is produced by the longer waves 



* Just as this proof is being returned to the press, a noteworthy series of findings 

 has been reported by a whole group of workers of the Biological Division at Oak 

 Ridge (see Rec. Genetics Soc. Amer., 1951, and the 1951 Cold Spring Harbor Sym- 

 posium Quant. Biol.) showing the influence of several different kinds of chemicals, 

 as well as of physical conditions, in modifying the effectiveness of ionizing radiation 

 in the production of radiation damage of varied kinds, including gene mutations 

 and chromosome changes, in diverse organisms. 



