470 RADIATION UIOLOGY 



3-11. lMI()T()!{i;A("ii\ A'i'iOX OK TIIK INDITTIOX I'ROCESS 



()!•■ iMiAci'; cAiuui;!) l.vs()(;l•:^•I(•Al-l.^■ 



As tliscoxcicd l)y liWolt tt al. (!•.).")()) cells of Bdcillii.s ntcydHwriiini 

 carryiiifj; lysofrciiic phafj;es can he induced to lyse by ultraviolet irradia- 

 tion. This cITcct of ultraviolet can also be photoreactivated, i.e., the 

 carrying bacteria will not liberate the phage if exposed to the photo- 

 reactivating light after the inducing exposure to ultraviolet (Jacob. 

 1950). 



Latarjet (19") lb) has shown that induction of the same organism is also 

 produced by X rays, and has announced (1951a, b) that the inducing 

 activity of X rays could be counteracted by photoreactivation; this result 

 is surprising since in all other cases X-ray damage has been found to be 

 at most very slight l}- photoreactivable. 



There is a reason to suspect that the apparent reversal of X-ray induc- 

 tion may not be photoreactivation. It has been found that E. coli B cells 

 irradiated with a dose of the photoreactivating light which does not kill 

 the bacteria and then infected with active phage T2 lose the ability to 

 produce phage. The effect observed by Latarjet (and possibly also by 

 Jacob) might be similar, and might consist in the loss of the ability of the 

 induced phage to grow in the illuminated bacteria rather than in a photo- 

 reversal of the induction (Dulbecco and Weigle, 1952). 



4. PHOTOREACTIVATION OF PLANT VIRUSES 



Bawden and Kleczkowski (1952) observed photoreactivation of tobacco 

 necrosis virus in French bean and of tomato bushy stunt virus in Nico- 

 tiana glutinosa. As with bacteriophages, the effect was obtained only 

 when the ultraviolet-irradiated virus was inoculated into the leaves, 

 which were in turn illuminated with white light. .\ virus preparation 

 whose activity was reduced to about 1 per cent could be restored by the 

 light to 4-10 per cent. No effect was observed in tobacco mosaic virus 

 inoculated into leaves of A'', glutinosa. 



The different photoreactivability of these three viruses may suggest a 

 correlation with their relative content in nucleic acid, which in tobacco 

 mosaic virus is about one-third of that present in the other two viruses 

 (see also Sect. 3-4). 



5. PHOTOREACTIVATION OF BACTERIA 



Extensive observations (Kelner, 1949b, c; 1950a; Novick and Szilard, 

 1949) have been performed on photoreactivation of bacteria, attention 

 having been focused almost exclusively on the bacterium E. coli B/r 

 (Witkin, 1946); a few observations have been made on the strains B and 

 Kr2 of the same species. 



