MECHANISMS OF MUTATION PRODUCTION IN MICRO-ORGANISMS 



SENSITIVITY TO INDUCED MUTATION AT DIFFERENT 

 STAGES OF GROWTH 



It was also noted in these experiments, that the sensitivity to induced muta- 

 tation varied with the period of preHminary incubation in medium {see 

 Table II), and this suggested that such changes might be related in some 

 way to the different stages in the nuclear division cycle as had been found 

 in higher organisms (for review see Sparrow*). Fortunately, the first 

 nuclear division is nearly synchronous in Streptomyces spore suspensions 

 (McGregor), and with the aid of parallel cytological observations it has been 



60 

 50 

 UO 

 30 



zo 



10 







30 

 SO 

 70 



eo 



50 



w 



30 

 20 

 10 



Figure 1. 

 •2 '/. 10" spores 



per 



il of 



Inoculum : 

 medium. 

 Incubation : 7 hours first day, refrigerated 

 overnight, 7 hours second day ; samples 

 from 7-hour suspension irradiated both 

 before and after the refrigeration (with 

 ultraviolet the storage caused an increase 

 from 18-5 to 20-4 per cent mutants, with 

 X-rays a decrease from 24-0 to 9-4 per 

 cent) ; suspension agitated at intervals 

 throughout incubation. 

 Growth : peak proportions of strands with, 

 3 to 5 nuclei =37 per cent at 5 hours, 

 6 to 10 nuclei = 58 per cent at 6 hours (40 



per cent at 5 hours) ; 

 higher numbers difficult to score with 

 accuracy. 

 Irradiation : samples diluted 1/10- with 

 saline before irradiation ; ultraviolet 200 

 ergs per mm-. X-rays 8,000 r. 

 Survival : per cent at 0, 1, 2, .... 14 hours, 

 for ultraviolet— 3, 5, 3, 3, 8, 39, 50, 78, 69, 



87, 85, 91, 81, 84, 100. 

 for X-rays— 17, 8, 3, 4, 18, 42, 58, 10, 51, 

 64, 73, 67, 59, 92, 66. 

 Unirradiated controls showed 0-5 per cent mutation and 100 per cent survival at 0, 7, and 



14 hours. 

 Each point in the curves for ultraviolet and X-ray mutation curves was based on approxi- 

 mately 400-1,200 colonies. 



to 



32 \ 

 16 \ 

 8 I 



1 



3 15 6 7 



9 10 11 12 13 11 



possible to study in detail the sensitivity changes occurring during this 

 division and the subsequent stages of germination and mycelial growth. 



The results of a typical experiment are shown in Figure 1. A spore sus- 

 pension was incubated in medium over a 14-hour period, samples being 

 taken at hourly intervals. One part of each sample was irradiated with 

 X-rays (approximately 8,000 r) and another with ultraviolet (200 ergs per 

 mm-) these being plated at a range of dilutions, while a third was fixed and 

 later stained for cytological study. For the sake of convenience the incuba- 

 tion was interrupted after 7 hours and the suspension stored in the cold 

 overnight. Samples taken before and after the storage, and treated in the 

 usual manner, showed that the interruption had not caused any major 

 change in response. 



330 



