300 Hubert P. Yockey 



inactivatioii of haploid yeast is exponential for x-rays and sigmoid for ultra- 

 violet. Anderson (16) has irradiated two biochemical mutants of strain B of 

 E. coli with x-rays, namely, the streptomycin dependent strain and the purine- 

 less strain. An exponential survival curve is obtained in oxygen while a sigmoid 

 curve is obtained in nitrogen for each strain. Hollaender, Baker, and 

 Anderson (17) have discussed the effect of oxygen and other chemicals on the 

 x-ray sensitivity for mutation production and survival. 



Hollaender and Stapleton (18) have shown that many types of survival 

 curves may be obtained ranging from exponential to a very pronounced sigmoid 

 shape depending on the experimental conditions. 



Stapleton, Sbarra, and Hollaender (7, 19) have studied the nutritional 

 aspects of survival of bacteria from ionizing radiation. They showed that the 

 B/r strain of £". co// grown on a complete medium such as nutrient broth exhibited 

 radiation-induced requirements for nutritional factors. They presented some 

 evidence showing that such bacteria are not stable auxotrophic mutants. 



The dependence of survivorship on nutritional factors is explained by 

 ZiRKLE and Tobias (20) from hit-theory concepts. They state that : 'Accordingly, 

 the number n of essential sites in the haploid chromosome set might vary with 

 the composition of the medium; in general, one would expect that the richer 

 the medium the fewer would be the observed number of essential sites. On 

 the other hand, if the 'inactivation' of a 'site' is not a mutation, but a gross 

 change in chromosome state or configuration, the number of sites would be 

 independent of the composition of the medium.' 



The interpretation of these results given by the authors quoted loses some 

 force since essentially the same results are found for viruses by Friedewald 

 and Anderson (21), iDy Luria and Exner (22), and by Dale (23). The explana- 

 tion offered by Luria and Exner is based on a two-fold action of the radiation, 

 a direct and an indirect effect. 



The case where two deleterious influences operate simultaneously is interest- 

 ing. Wood (24, 25) has studied the x-ray survival of haploid yeast as a function 

 of temperature. The curves show a distinct tendency to be concave downward 

 for temperatures between 45°C and 55°C. He finds a 'softening' or 'memory' 

 of exposure to temperature and x-rays for the action of the other. Uretz (15) 

 finds very little 'softening' in his study of the action of x-rays and ultraviolet 

 on haploid yeast. We are not aware of a study of the ultraviolet survival as a 

 function of temperature, although such data would be of importance to complete 

 knowledge of these effects. 



Gray (26) has pointed out recently that a view is gaining general acceptance 

 that a site may be inactivated by a single fast electron, but not by the absorption 

 of a single photon. The site mentioned by Gray is interpreted as a nucleotide 

 pair in the present paper. The action of the deleterious effect may be, partly 

 at least, to throw the nucleotide pair into an excited tautomeric form. In such 

 a form it may be more easily damaged by a successive interaction. The extent 

 to which this occurs may very well depend on the chemical environment. At 

 any rate, for the present purposes, there is reason to believe that J{X) may be 

 represented by a polynomial in A. The higher order terms represent higher 

 order reactions. 



In that event, it is possible to begin to understand why an exponential 



