PHOTOREACTIVATION 479 



has been determined in detail by Kelner (1951); it has ah-eady been dis- 

 cussed (Sect, o-o) together with the bacterial action spectrum. 



In Neurospora crassa, Goodgal (1950) has observed that the survival of 

 microconidia irradiated with ultraviolet is affected by photoreactivation, 

 which produces an increase in the number of active conidia with a con- 

 stant reduction in ultraviolet dose. The freciuency of morphological 

 mutants among survivors has also been studied and found to be similarly 

 reduced by photoreactivation. The ultraviolet dose reduction is of the 

 same order of magnitude for lethal and mutagenic action, since a given 

 survival corresponds to a constant fraction of mutants, independently of 

 the presence or absence of photoreactivation. This result is therefore 

 similar to that obtained by Xovick and Szilard (1949) in E. coli (Sect. 5-7). 

 Goodgal attributes these results to one of two possibilities: (1) killing of 

 microconidia by ultraviolet is due to a lethal mutation, and light decreases 

 killing by hindering the occurrence of mutations; or (2) killing and muta- 

 tions are produced bj^ a common mediator formed under ultraviolet 

 treatment and destroyed by the light. 



Brown (1951) studied the effect of photoreactivation on the reversion of 

 the inositol requirement in .V. crassa by irradiating mononucleate micro- 

 conidia with ultraviolet, and found that the fraction of mutated conidia 

 among the survivors is considerabl}^ less after photoreactivation. This 

 result cannot be due to lack of photoreactivation of inactive mutants, 

 since after photoreactivation the absolute number of mutants increases. 



7. PHOTOREACTIVATION IN YEAST 



Photoreactivation in yeast has been demonstrated by Kelner (1949a) 

 and by Swenson and Giese (1950). The latter authors have shown that 

 the mechanism of enzymatic adaptation to galactose fermentation in 

 Saccharomyces cerevisiae is damaged by ultraviolet and that this effect also 

 can be partly undone by exposing the cells irradiated with ultraviolet to a 

 strong white light. Several points in an action spectrum for the ultra- 

 violet destruction of adaptability have given results compatible with the 

 assumption that the radiation acts on nucleic acid (Swenson, 1950). 



8. PHOTOREACTIVATION IN PROTOZOA 



Kimball (1949) and Kimball and Gaither (1950, 1951) have studied 

 photoreactivation in Paramecium aurelia by using monochromatic radi- 

 ation of wave length 2804 and 2650 A in the majority of the experiments. 

 In this organism irradiation with ultraviolet has a number of effects: 

 I'etardation in cell division, killing of a fraction of animals before autog- 

 amy, reduced vigor after autogamy, and morphological changes in the 

 structure of the macronucleus. 



