548 RADIATION BIOLOGY 



pathway which is of considerable duration and which is susceptible to 

 being influenced and studied by chemical means, was not in itself a new 

 one ; for this had already been made clear by entirely different means in a 

 remarkable series of studies carried out by Stone and Wyss and their 

 co-workers, beginning in 1947. 



13. MUTAGENESIS BY ULTRAVIOLET TREATMENT OF THE MEDIUM 



The crucial discovery in these studies (Stone, Wyss, and Haas, 1947) 

 consisted in the finding that ultraviolet irradiation of the culture medium 

 prior to immersion of the organisms in it resulted in a manyf old increase in 

 the frequency of mutants. This effect, obtained first with Staphylococcus 

 aureus {Micrococcus pyogenes), was later extended to E. coli (Haas et al, 

 1950) and to Neurospora (Wagner et al, 1950). It was shown not to be 

 caused by a selection favoring the surAdval or growth of the given mutants 

 but by an actual production of mutations (Stone, Wyss, and Haas, 1947; 

 Stone et al. , 1948) . Pretreatment of the medium with a mustard mutagen 

 was likewise shown to cause mutations in organisms (Staphylococcus) later 

 inoculated into the medium (Wyss, Stone, and Clark, 1947). In these 

 cases, then, the mutagenic pathway leading from the atomic activation 

 or other primary chemical change to the gene mutation is by no means 

 spatially restricted. Moreover, it also has a considerable latitude in 

 time since the decay of mutagenic activity of the treated medium occupies 

 an interval of the order of hours. The rate of decay proved to be tem- 

 perature dependent, like an ordinary chemical reaction (Stone, Wyss, and 

 Haas, 1947). 



In a chemical attack on the processes involved, it was shown (Stone, 

 Wyss, and Haas, 1947) that the effect could be produced by irradiation of 

 a solution of only the amino acid constituents of the m.edium but not by 

 irradiation of a solution of only the mineral constituents. It was then 

 found that amino acids could be caused to become mutagenic by treat- 

 ment with hydrogen peroxide instead of with ultraviolet (Wyss, Stone, 

 and Clark, 1947). That this was the clue to the ultraviolet influence on 

 the medium was apparent from the finding that the amount of mutational, 

 as well as of destructive, action of the irradiated medium was approxi- 

 mately proportional to the amount of hydrogen peroxide which had 

 originally been produced in the medium, as measured by the rate of 

 production of hydrogen peroxide in ultraviolet-irradiated water (Wyss 

 et al, 1948). Nevertheless, most of the hydrogen peroxide itself had 

 apparently disappeared before inoculation (although these tests may have 

 been somewhat influenced by the organic substances present) . However, 

 even in cases in which all the hydrogen peroxide seemed to have vanished, 

 the activity of the irradiated medium was by no means abolished. 



That the active medium still contained peroxides, quantitatively equiv- 



