MANNER OF PRODUCTION OF MUTATIONS 579 



a slightly more than additive effect on the frequency of visible mutations. 

 This means either that in this organism, unlike the other two, ultraviolet 

 actually promotes the production of aberrations by X rays (assuming the 

 excess mutations to represent aberrations) or that its mode of producing 

 point mutations is so different quaUtatively from that of X rays that the 

 combination of both effects is more potent than an equivalent increase 

 of dose on the part of either agent alone would be. On the other hand, 

 mustard, in the preliminary tests on Aspergillus, did not work synergistic- 

 ally with X rays when given as a pretreatment but only additively, as 

 though the mutagenic processes initiated by these two agents were either 

 alike or quite independent. That they were not entirely alike had, how- 

 ever, been shown by Swanson and Goodgal's (1947; see also Swanson, 

 1952) finding in Aspergillus that mustard and ultraviolet exert a very 

 marked synergism with one another, far stronger than that found between 

 X rays and ultraviolet. It therefore seems indicated by this group of 

 results that mustard and X rays act, for the most part, through some- 

 what different and noninteracting chains of processes. 



It is natural, because of the practical value which the results would 

 have, that more has been done in finding ways to hinder than ways to 

 promote mutagenesis by ionizing radiation. The light thrown in recent 

 years on the role played by oxygen and by the active oxygen-carrying 

 radicals and molecules derived from water on the production of "dam- 

 age," mutations, and chromosome breaks by radiation has opened up new 

 pathways for such work. This is not the place to review the various 

 findings which have been made regarding the prevention or diminution of 

 radiation damage by such means as anoxia or cysteine, or the chemical 

 studies which led up to these applications. A systematic large-scale 

 study has recently been made by HoUaender and his co-workers (Burnett 

 et al., 1951; HoUaender, Stapleton, and Burnett, 1951; HoUaender, 

 Stapleton, and Martin, 1951; HoUaender, Baker, and Anderson, 1952; 

 HoUaender, 1952) of organic chemicals which, given before exposure, afford 

 protection against radiation damage; E. coli was the organism most used, 

 but the results were checked on various other forms. It was then found 

 by E. H. Anderson (unpublished, cited by HoUaender, Baker, and 

 Anderson, 1952) that the same agents also serve to protect E. coli against 

 the mutagenic effects of ionizing radiation as against its killing action. 

 The extent to which these substances may also give protection against 

 ultraviolet effects has not yet been determined. 



The protective compounds thus far found were classified into four main 

 groups : 



1. Sulfhydryl compounds, such as cysteine, BAL, and mercapto- 

 carboxylic acids, with distinct, virtually immediate effects even at 

 0.001 M, without prior incubation in a medium containing them. 



