MECHANISMS OF MUTATION PRODUCTION IN MICRO-ORGANISMS 



Storing overnight in the cold. Samples taken each day were tested for 

 spontaneous mutation and for sensitivity to X-ray and ultraviolet induced 

 change. Most of the growth was probably completed in the first 14 hours. 

 Under these conditions the spontaneous mutations increased continuously 

 with incubation {see Table IX). X-irradiation however induced only 1 or 

 2 per cent of additional mutants whereas ultraviolet was a remarkable 

 effective mutagen despite the multinucleate state, inducing from 15 to 



Table IX. — Sensitivity to X-rays and to ultraviolet in the uninucleate and multinucleate states 

 (Dose, 8,000 r. and 400 ergs per mm^ respectively) 



Inoculum : 2 X 10' spores per ml. 



Incubation : in medium ; intermittent 7 hours out of each 24 ; stored in cold between 

 incubations. 



Growth : all spores have germinated and grown into a multicellular mycelium (about 

 30 nuclei per infective unit) by 7 hours. The suspension probably reaches saturation 

 shortly after. 



55 per cent mutants {see Table IX). Thus, a state seems to have been 

 achieved in which the sensitivity to the mutagenic action of the two agents 

 differed widely. In addition there appear to have been significant fluctua- 

 tions in sensitivity to ultraviolet induced mutation, and it would seem that 

 such sensitivity must have been very much dependent upon uncontrolled 

 physiological variables. 



Finally, spore suspensions in medium were bubbled with nitrogen and 

 incubated, over a 24-hour period. During the treatment almost all nuclear 



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