118 RADIMIo.N MIOLOGY 



is photorcaclivatxHl to al)()iit the .same cxtciil. Xo spcculal ions con- 

 cerning the nature of the uongenetic nuclear daraage are given, hut it 

 would seem that it may be of a gross nature, and hence may involve a 

 number of genes. The observation of fiist-order kinetics indicates that 

 the ultraviolet quantum is capable of producing this effect. 



These observations with yeast, and especially with Ncuronpora, indicate 

 that the damage causing lethality is damage to the nucleus, i.e., to the 

 genetic apparatus. Furthermore, the Ncuroapora results indicate that 

 recessive lethal mutation is of relatively minor importance in the inacti- 

 vation of multinucleate cells by irradiation. Since there is an increasing 

 amount of evidence that bacterial cells of certain species may be multi- 

 nucleate, at least at certain stages of development of the culture, a similar 

 situation may obtain in bacteria. This has been indicated by the results 

 of Stapleton (1952), who found differences in X-ray resistance and in the 

 kinetics of inactivation which were correlated with differences in the 

 mean number of nuclei of E. coli B/r cells at different stages of the growth 

 cycle. 



Witkin (1951), however, found resistance of E. coli B/r to ultraviolet 

 to be lowest in growing cells, greater in resting cells, and highest in cells 

 in the lag phase. These differences in resistance did not parallel differ- 

 ences in the average number of nuclei — 4,2, and 4, respectively — for the 

 three stages of the growth cycle and seem incompatible with the recessive 

 lethal-mutation hypothesis of radiation killing. Furthermore, the cor- 

 relation between nuclear number and the size of lactose negative sectors 

 in colonies derived from cells which survive irradiation strongly suggests 

 that nuclear segregation is partially responsil^le for the sectored colonies. 

 If this is actually the case, it is questionable if ultraviolet killing is nuclear 

 at all since, as Witkin points out, wdth survivals as low as 10"'' as in her 

 experiments, the probability of a surviving cell having more than one 

 viable nucleus would be very low. 



Dale (1940, 1942) has suggested that the bactericidal effects of irradia- 

 tion may not involve lethal mutations but rather the inactivation of 

 enzymes. Although objections to this hypothesis have been made on the 

 basis of the relative insensitivity of enzymes irradiated in vitro, Dale has 

 shown that dilute solutions of purified enzymes are quite sensitive to 

 radiation, and, indeed, this sensitivity is the l)asis for his suggestion that 

 enzyme inactivation may be a factor in the bactericidal effects of irradia- 

 tion. Barron et al. (1949) have showni that the sulfhydryl-containing 

 enzymes are particularly sensitive to ionizing radiations. It is known 

 that dilute solutions of enzymes are inactivated primarily by an indirect 

 effect of ionizing radiations and that a great variety of compounds protect 

 the enzyme against radiation by competing for the highly reactive prod- 

 ucts formed from water. It would appear that a similar competitive pro- 

 tection would occur within the cell, since a great variety of prolc'iiis are 



