MUTAGENS OF POTENTIAL SIGNIFICANCE 217 



seems improbable for caffeine. It should be pointed out, as I think 

 Dr. Novick did this morning, that there might be a relationship with 

 time, but it was, nevertheless, essential that bacterial growth occur 

 during the exposure to caffeine or else one would not get mutagenesis; 

 likewise, in his chloramphenicol experiments, there was no muta- 

 genesis. 



Novick: It is a simple enough experiment, I would imagine, to add 

 P^- to see if it is incorporated into the DNA of spermatogonia. If it is 

 incorporated, they are exactly like our bacteria, and mutations would 

 be induced at a constant rate per hour. 



Atwood: Kubitschek (35) published to the effect that caffeine- 

 induced mutation rate is proportional to growth rate, but I don't think 

 you believe that. 



Novick: He said "under one circumstance." 



Atwood: When the growth rate is limited by the carbon source. I 

 think that was glucose. 



Goldstein: To what extent do you think that your results may be 

 related to the growth limitation by tryptophan? 



Novick: We tried it for a wide variety of growth factors, including 

 limitation by lactate and ammonia. In all these cases the induced 

 mutation rate was a constant per hour. With bacteria, one can define 

 two states with respect to growth: a growing state and a nongrowing 

 state. The extreme nongrowing state is the spore or the resting bacter- 

 ium. Here one can induce mutations with things like irradiation and 

 nitrous acid, and not with caffeine. In growing bacteria, there is some 

 kind of limitation, whether it be nitrogen source, amino acid, energy 

 source, etc. Growth can be limited to very low rates and I think that 

 such bacteria are very much like many of the cells in the higher 

 organisms. 



Glass: Is that growth apart from division? 



Novick: Growth in the sense I use it is always accompanied by 

 division. 



Glass: That's what I thought in bacteria, but we may be faced with 

 a different type of growth in the germ line. 



Novick: I think the important point in all of our experiments is that 

 where DNA is being synthesized, where P^^ is being incorporated into 

 the DNA, the system is susceptible to mutagenesis. 



Goldstein: I would point out that in the male the spermatogonia, 

 stem line spermatogonia, are growing slowly. This is accepted. In 

 that system, it doesn't matter whether you talk about absolute time or 

 biological time. It amounts to the same thing in the sense that there 

 is a mean generation time. The real question is whether the time scale 



