478 KADIATIOX UIOLOGY 



since llif latter authors used lower (loses in their experiments. This 

 explanation does not account for all the dilTerences, however, since the 

 fractions of mutants anions survivors in points correspondinfj; to the sur- 

 vival vahies observed by Kelner in the absence and in the presence of 

 photoreactivation differ, according to Demerec and Latarjet (1946), by a 

 factor of 10 or more, less mutations being present at the lower dose. 



The etTect of photoreactivation on the so-called " zero-point mutations" 

 seems not sufficiently documented in view of the fact that detection of 

 these mutations iii\-ol\-es infection with phage of a very large number of 

 bacteria inactivated with ultraviolet but able to adsorb the phage. 

 Under these conditions the multiplicity of phage infection may become 

 too low, so that a considerable fraction of bacteria may divide before 

 infection. 



Newcombe (1950) and Newcombe and Whitehead (1950) have observed 

 that the mutagenic effect of ultraviolet on E. coli B/r (streptomycin- 

 resistance mutants and color-response mutants on mannitol-tetrazolium 

 agar) is particularly strong at low doses of ultraviolet. At these low doses 

 the reversal of the mutagenic effect by photoreactivation is also strong. 

 At a dose of 500 ergs/mm- more than 90 per cent of the potential mutants 

 fail to appear after photoreactivation. At larger doses the mutations 

 produced by ultraviolet alone do not increase with dose, and their photo- 

 reactivability tends to zero. 



The results show that the effect of photoreactivation on these mutations 

 can be approximately described on the basis of the dose-reduction prin- 

 ciple, as in the case of Novick and Szilard (1949), although the mutagenic 

 effect of ultraviolet is reduced by photoreactivation to a greater extent 

 than the killing action as determined by Kelner (1949c). This difference 

 may not be significant since the amount of reduction of the killing action 

 has not been determined inider the same conditions. 



6. PHOTOREACTIVATION IN STREPTOMYCES AND FUNGI 



Photoreactivation was first discovered in spores of Streptomyces griseus 

 irradiated wdth ultraviolet (Kelner, 1949a). Spores suspended in saline 

 or distilled water are exposed to ultraviolet and then treated with visible 

 light Avith the following results: (1) the ratio of the number of active spores 

 after photoreactivation to the number of active spores before photoreacti- 

 vation increases with the ultraviolet dose; (2) with light of constant inten- 

 sity the number of active spores tends to a maximum value with time of 

 illumination ; (3) increasing the intensity of the reactivating light increases 

 proportionately the rapidity of recovery within certain limits; and (4) the 

 rapidity of recovery increases with the temperature existing during light 

 treatment, up to temperatures of 50°C. 



The action spectrum of photoreactivation of S. griseua spores (Fig. 12-7) 



