546 RADIATION BIOLOGY 



consideration that in some of the work, done before the reparative 

 effect was known, the ultraviolet treatment was probably accompanied 

 by enough light of longer wave lengths to cause some of the falling off of 

 the slope of the frequency-dosage curve that was observed at the higher 



doses. 



The temperature coefficient of the reparative process, being like that 

 for ordinary chemical changes, suggests that the process might be influ- 

 enced or even imitated by chemical changes other than those dependent 

 on light. That this is the case has been shown by independent studies on 

 ultraviolet mutagenesis in Aspergillus by Swanson and Goodgal (1950) 

 and in Drosophila pole cells by Meyer and Muller (1952) and Edmondson 

 and Meyer (1952) and on survival in E. coli by Hollaender (cited by 



Howard, 1950). 



In the studies on Aspergillus it was found that, when dinitrophenol is 

 applied either before or after irradiation with ultraviolet, there is a con- 

 siderably higher mutation frequency than without it. Since this sub- 

 stance, although promoting some oxidations, retards those metabolic 

 processes on which the rate of development depends, it is conceivable that 

 a reparative action on ultraviolet mutagenesis which was regularly 

 exerted (even in the absence of reparative light) might under the influ- 

 ence of dinitrophenol be rendered less active. In this case the effect was 

 reported not to be paralleled by one on survival, but, since it could be 

 demonstrated only at low doses of ultraviolet, where the curve was rising, 

 not so much effect on survival would be expected anyway. Potassium 

 cyanide, also a metabolic inhibitor, but through a different pathway, was 

 found to have no such effect in Aspergillus (although, as will be noted in 

 Sect. 13, cyanide by itself causes a marked rise in the mutation rate in 

 Neurospora) . 



In the studies on Drosophila it was found that, even in the absence of 

 significant amounts of reparative light, the presence of oxygen, as 

 opposed to nitrogen, during the ultraviolet irradiation (posttreatments 

 of this kind having not yet been tested) resulted in a considerable rediic- 

 tion in the induced-mutation frequency. Second, the temperature after 

 irradiation exerted an influence also, even without light, in the same direc- 

 tion as it had been found to do with light in the " photoreactivation " of 

 microorganisms, i.e., there was more repair at higher temperatures. 

 Similarly, in HoUaender's recent work with E. coli it was found that heat, 

 even in the absence of hght, tends to reduce the damage to survival caused 

 by a previous ultraviolet treatment. 



In all these cases it seems most likely that some interference with 

 normal metabolic processes blocks a reparative action, on the products 

 of ultraviolet, which ordinarily occurs even without light. However, it' 

 would be equally permissible, from a purely descriptive point of view, to 

 refer to the results as showing that there is an enhancement of the muta- 



