544 RADIATION BIOLOGY 



correspondingly further when higher temperatures exist during the 

 period of apphcation of the reparative hght. However, p, the size of the 

 fraction which is subject to repair, is not altered thereby. In Strepto- 

 myces, Kelner (1952) found that the speed of the reaction is raised about 

 three times for each rise of 10° between 20° and 40°C (i.e., Qio = 3), a 

 value similar to that found in the case of most ordinary chemical reac- 

 tions. He also found that the repair of ultraviolet inactivation by light 

 occurs even in dry spores of Streptomyces and therefore takes place within 

 the cell. On the other hand, Hill and Rossi (1952) found such repair 

 impossible in bacteriophage when the damaging irradiation of them had 

 been carried out in the dry state. Hence certain conditions can alter the 

 ratio of p to q. 



The action spectrum of the reparative light was found by Kelner to be 

 different for different species of microorganisms, and it appeared for one 

 species studied to correspond approximately with the absorption spectrum 

 of cytochrome oxidase. This finding is of special interest in view of the 

 fact that the activity of cytochrome oxidase is under certain conditions 

 furthered by light (as shown by activation of this enzyme by light after 

 it has been inactivated by carbon monoxide). Since the activity of this 

 and related enzymes such as catalase tends to reduce the amount of 

 oxygen, hydrogen peroxide, and their active derivates (e.g., OH) present 

 in the cell, it is evident that, in so far as ultraviolet produces its damage 

 via the production of the latter substances (see Sect. 13), the visible light, 

 if it furthered the activity of these enzymes, would tend to counteract the 

 damaging action of the ultraviolet. In line with this, the recent observa- 

 tion of Latarjet (cited by Howard, 1950) should also be noted, that in a 

 strain of E. coli the administration of catalase to the cells greatly 

 increases the reparative effect of visible light on ultraviolet inactivation. 



For the bacteriophages studied, which do not seem to contain such 

 enzymes, reparative light is effective only after the phage has become 

 attached to the bacterium (Dulbecco, 1949, 1950). However, it has 

 been found in the work of Haas et al. (1950) that light can under certain 

 circumstances have some reparative influence even in the absence of any 

 enzymes or coenzymes; for both the killing and the mutagenic effects of 

 ultraviolet-treated medium (pp. 548-552) were reduced by visible illumi- 

 nation of the medium itself, subsequently to the ultraviolet irradiation 

 and prior to immersal of the bacteria in it. This result was obtained not 

 only with nutrient broth but also with a synthetic medium (Gladstone's) 

 containing only certain amino acids, vitamins, and minerals. They sug- 

 gest, in agreement with their other findings, that the result may be due to 

 a destruction of unstable molecules such as peroxides by the visible light, 

 a process which could take place, although much more slowly, even in the 

 absence of catalysts. 



The reparative action of light on ultraviolet damage was first extended 



