MECHANISMS OF MUTATION PRODUCTION 

 IN MICRO-ORGANISMS 



Howard B. Newcombe 



Atomic Energy of Canada Limited, Ciialk River, Ontario, Canada 



A PECULIARITY ill the action of X-rays on the spores of one of the 

 Actinomycetes was noted by Kelner^ in 1948. It appeared that high doses 

 of X-rays altered the capacity of the spore svispension for further mutational 

 response, so that still higher doses caused no corresponding increase in the 

 proportions of mutant spores found among the survivors. In some experi- 

 ments the proportion of mutants reached a plateau and remained unaltered 

 as the dose was increased, while in other experiments the mutant frequency 

 rose to a maximum and then declined as the dose was raised still further. 



This observation was of interest because it indicated a similarity in the 

 actions of X-rays and ultraviolet light which had not previously been 

 suspected. The dose-mutation curves for ultraviolet had been known for 

 some time to be non-linear in a wide range of experimental organisms, 

 showing either a plateau, or a peak followed by a decline ; but in the same 

 materials, where the comparison was made, the corresponding response to 

 X-rays was notably linear. Streptomyces is exceptional in that it is the one 

 organism so far studied in which a similar saturation or impairment of the 

 capacity for induced mutation has been observed at high doses, with both 

 ultraviolet and X-rays. 



For this reason we decided to carry out a further study of the radiation 

 response relationships in another strain of this organism, and I shall describe 

 certain of the experiments into which this has led us, and the bearing which 

 the results appear to have on the problem of the origin of the induced 

 mutations. 



materials 



It should be emphasized that Streptomyces is exceptionally well suited to 

 quantitative experimentation. The spores are uninucleate, and suspensions 

 can be prepared in which there is no clumping. The induced mutations, 

 which affect both colony morphology and colour, are numerous and clearly 

 recognizable ; and to observe them one has only to plate suitable dilutions 

 of the treated suspensions and examine the resulting colonies. It is thus 

 possible to plan experiments in which large numbers of mutants can be 

 scored, and where necessary the experiments can be repeated many times, 

 often on successive days with the scoring being done the following week. 

 (For media, and methods of handling, see Newcombe^.) 



The induced changes do not revert to the wild type, except as rare events 

 in certain mutant lines. However, many of them give rise to a range of 

 further mutational changes with very high frequencies, and for this reason 

 the initial changes might be described as mutations to a 'variegated' form. 



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