MUTAGENESIS 85 



Auerbach: I see. They couldn't explain this discrepancy, though. 



Freese: Oh, no. 



Benzer: You can have some r+ in a plaque and it may not look 

 mottled. I wonder whether you picked the r plaques and replated them 

 to see if they contained a minority? 



Freese: No. This analysis has nut been done in the detail to rule 

 out this possibility, but, since we get the straight line in this plot — 



Benzer: That factor might tend to make the number even higher. 

 If some of what appear to be pure r plaques actually contain some r + 

 particles, the number of mottled would tend to be underestimated. 



Goodgal: Might it mean that the curve is the result of compensation, 

 and the straight line doesn't mean anything. 



Freese: No. The straight line does mean something, but it could be 

 that the whole straight line has to be shifted. 



Glass: Would you mind stating your final conclusion again? I'm not 

 sure that I followed the derivation of that from the data. 



Freese: One can draw two conclusions: first, one would expect that 

 the longer one treats phages by a mutagen, the more pure mutants one 

 should get, because, occasionally, one kills that strand which does not 

 have the r mutation in it. That is clearly indicated by this curve. The 

 second question is whether, by extrapolating to zero treatment, one 

 finds only mottled plaques or whether one still gets some r plaques. 

 Although the examination has not been done in the necessary detail, I 

 think all our observations indicate that even at zero time, you would 

 not get only mottled plaques but also r plaques. 



There is one objection. One should really pick the plaques for all 

 points along the curve and replate them, even those which look like 

 pure r plaques. We have done this for a few points and then found in 

 agreement with the above statement that most r plaques contained only 

 r phages and not a mixture of both wild type and r phages. 



Auerbach: That could account for your missing 30 per cent. If that 

 were so, it would mean you missed 30 per cent. 



Freese: Yes. I don't think that is very likely. 



Benzer: The curve drops rather rapidly. Does that imply that a hit 

 somewhere else is preventing development of one of the chains? 



Freese: Yes, I would say so. 



Benzer: It goes more rapidly than you would expect if it required 

 another r hit to knock out a mottled plaque. 



Freese: Yes. 



Auerbach: This problem comes up in Kaudewitz's paper (38), 

 which I found somewhat puzzling. It is the only paper which 

 I know on treatment of bacteria with nitrous acid. He tested 



