MUTAGENESIS 119 



TABLE 2 



Numbers of single-site mutations, long deletions, and short deletions among 

 cysC mutants of different origin. Factor by which number of mutants is larger 

 in treated series than in control series; 2AP, xl25; UV, xlO; X-rays, x20; neutrons, 

 xll; NaNOs, xl2. 



Demerec: Long, as I remember. We have three short deletions in 

 X-rays, and we have three short deletions in total, with spontaneous 

 and ultraviolet. Now, this would not be very significant, but we sent 

 histidine-induced mutants to Phil Hartman at Johns Hopkins. He is 

 interested in the analysis of histidine loci, and has analyzed a consider- 

 able number of histidines. He tells me that he finds short deletions 

 present more frequently among x-ray induced mutants than among 

 spontaneous. 



Lederherg: You mentioned the figure, 4 per cent. Is that the fraction 

 of short deletions among all deletions? 



Demerec: No, that is the fraction of the deletions among all muta- 

 tions found in 23 gene loci, not including cysC. 



Benzer: Is there some other feature that distinguishes the UV and 

 X-ray ones from the spontaneous? 



Demerec: We cannot say. 



Benzer: Could they be spontaneous ones which have somehow been 

 enriched? 



Demerec: That seems very unlikely, because the same procedure — 

 the penicillin technique — was used to isolate auxotrophs in all the 

 experiments. In the UV experiments, as the table shows, mutants were 

 about ten times more frequent in the treated series than in the un- 

 treated controls. 



Goodgal: The deletions you were talking about, the long deletions, 

 were they selected in separate experiments? 



Demerec: Oh, no! They were found during an analysis of cysC 

 auxotrophs isolated in different experiments. 



