420 RADIATION BIOLOGY 



chemical events may be similar in the two orf^anisms, and involve the pro- 

 duction of a product sensitive to \isihle lijj;h1 w Inch subsecjuently initiates 

 ill llic two orjianisms two different series of reactions leadinji; t(j different, 

 ultimately lethal effects. It is possible that both mechanisms are oper- 

 ative in both strains witli the diricrence in sensitivity due to the single 

 gene mutation being a shift in the relati\(' jiroportion of effects of radi- 

 ation produced by the two mechanisms. With both B and B/r a non- 

 photoreactivable residue exists, making it seem necessary to postulate 

 in each strain an additional mechanism which is at least partially distinct 

 from the photoreactivable mechanism. 



To these mechanisms involving ultraviolet must be added still different 

 mechanisms for X rays since no photoreactivation occurs and, in strain 

 B, no filamentous forms are produced (Roberts and Aldous, 1949). The 

 X-ray effects, in turn, are subdivided into an oxygen-influoncod and an 

 oxygen-independent mechanism. As with the photoreactivable and non- 

 photoreactivable mechanisms of ultraviolet, it is not clear if the oxygen- 

 dependent and oxygen-independent effects of X rays produce the same or 

 different kinds of ultimately lethal damage to the cells. Thus the obser- 

 vations on these two closely related strains indicate that a variety of 

 mechanisms exist by which radiations produce bactericidal effects. 



In view of the ionization-density studies, it seems likely that different 

 mechanisms are involved in the inactivation by high-energy radiations of 

 spores and vegetative cells. 



A quite different mechanism by which radiation can cause the killing 

 of bacterial cells has been demonstrated by Lwoff et al. (1950) in studies 

 of lysogenic strains of bacteria. It was found that small doses of ultra- 

 violet will induce, in nearly all the cells, the formation of active bac- 

 teriophage resulting in lysis. Latarjet (1951) has shown that small doses 

 of X rays have a similar effect and, furthermore, that visible light reduces 

 the effect following both ultraviolet and X irradiation. Similar ultra- 

 violet treatment will cause phage production and lysis of E. coli K12 

 (Weigle and Delbriick, 1951) which E. Lederberg (personal communica- 

 tion) discovered to be lysogenic. 



A similar variety of mechanisms may be involved in mutation produc- 

 tion since it is known that mutations are induced by the photoreactivable 

 and nonphotoreactivable ultraviolet mechanisms and by X rays both in 

 the presence and in the absence of oxygen. In addition, it is becoming 

 apparent that different loci may respond quite differently to a particular 

 radiation, as evidenced by the different types of mutation-dose curves 

 and the differential response of two different mutations to X rays in the 

 presence and absence of oxygen. 



Many problems have been uncovered which, when more thoroughly 

 analyzed, promise to add much to the knowledge of the effects of irradia- 

 tion in bacteria. In the more detailed analysis of the problems, parallel 



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