DISCUSSION 



Two observations support the view that the non-linear X-ray dose- 

 mutation curves observed in Streptomyces are the result of a genuine satura- 

 tion or impairment of the capacity of individual spores to respond when 

 the dose is high : 



First, at doses where a further increase is ineffective in producing muta- 

 tions, it is also relatively ineffective in producing lethal changes. Second, 

 cell suspensions have been obtained by pre-incubation which are very 

 resistant to X-ray killing ; and in these the less-than-Iinear response to 

 increasing doses could not be accounted for in terms of differential killing 

 of cells which would otherwise produce mutant colonies. 



In addition, very considerable changes in sensitivity to X-ray induced 

 mutation have been observed following the first nuclear division, and also 

 as the result of various treatments affecting the physiological state at the 

 time of irradiation. As the capacity for induced mutation is evidently 

 dependent on the physiological state of the organism, it is not surprising 

 that high doses of radiation should alter this capacity for mutagenic response. 



REFERENCES 



1 Keener, A. J. Bad. 1948, 56 457. 



2 Newcombe, H. B. J. Gen. Microbiol. 1953, 9 30. 



^ Newcombe, H. B. and McGregor, J. F. Genetics, 1954, in press. 



* Sparrow, A. H. Atm. N.T. Acad. Sci. 1951, 51 1508. 



^ McGregor, J. F. J. Gen. Microbiol. 1954, in press. 



« Newcombe, H. B. Genetics, 1951, 36 570. 



" Newcombe, H. B. ibid, 1953, 38 134. 



8 WiTKiN, E. Cold Spring Harbor Symp. Quart. Biol. 1931. 16 357. 



DISCUSSION 



R. Latarjet : La chute du taux des mutations induites par les fortes doses de 

 rayons ultraviolets a un caractere tres general, conformement aux indications du 

 Newcombe. Aux cas qu'il a 

 mentionnes, jepuisajouter celui 

 que j'ai observe en obtenant les 

 premieres mutations qui aient 

 ete induites chez un virus* 

 (Figure2). II s'agissait de muta- 

 tions 'host range' obtenues chez 

 le bacteriophage T2 en irradiant 

 celui-ci pendant qu'il se multi- 

 plie au sein de son bote Escheri- 

 chia coli souche B. 



La courbe ci-jointe groupe les 

 points experimentaux obtenus. 

 EUe montre la chute tres brusque 

 des mutations induites a partir 

 d'une dose de 5.000 ergs mm"^. 



* Compt. rend. Acad. Sci. Paris, 

 1949,228 1354. 



3 It- 



Figure 2 



5 6 



jrlOOOerqs Tum^ 



339 



