Discussion 207 



rapid expression would seem difficult to account for if the units for 

 sensitivity were lost, during growth after the mutation to resistance, 

 simply by dilution. 



Hayes: I had visualized a state of affairs in which, after segregation, 

 the genes started functioning, producing the necessary system to manu- 

 facture new sites on the cell wall; and that you would have a hetero- 

 geneous population of cells at various stages, some of which had a certain 

 proportion of sites, and others a different proportion. It is just a matter 

 of chance; the more sites a particular cell had synthesized at any 

 particular time, the greater the chance that under the experimental 

 conditions it would be able to absorb that phage and be killed. 



Cavalli-Sforza: An incidental point is that I do not agree entirely with 

 the genetic map which you showed. Dr. Hayes. I think valine resistance 

 should be between T and L, according to my experience. You said that 

 valine resistance is expressed immediately and more rapidly than azide ; 

 do you find any difficulty in scoring for azide resistance in minimal? 



Hayes: No, but this is a technical point. I use sodium aspartate in 

 my minimal medium, and if I use m/1500 azide there is no difficulty at 

 all in scoring colonies of resistant segregants. This level suppresses the 

 growth of sensitive prototrophs, but allows resistant prototrophs to 

 grow. The colonies are smaller than on minimal agar without azide, but 

 this makes counting rather easier. The colonies come up after overnight 

 incubation at 37°. 



Fredericq: When you add your T6 phage to destroy the donor cells, 

 this is at a time when the two cells are sticking together. Do you some- 

 times observe a transfer of T6 from the donor into the recipient cell ? 



Hayes: This would be hard to observe directly, but by inference, no. 

 After this treatment one gets exactly the same number of recombinants, 

 within the experimental error, as one gets when the mating pairs are 

 simply separated, diluted and plated out. There is no reduction. 



One could assume that if the phage was transmitted, then it would 

 probably multiply and lyse the otherwise resistant cells, and this is 

 not so. 



Stacker: With regard to the time needed for expression of strepto- 

 mycin resistance, one can make an analogous experiment either in DNA 

 transformation or phage transduction. Dr. Hotchkiss and others have 

 described the results in the DNA experiment. In transduction experi- 

 ments, there is a considerable delay after the phage is added before the 

 cells first become streptomycin-resistant, and there is then a plateau 

 before the number of streptomycin-resistant clones begins to increase. 

 There is also a delay in the case of transduction of motility (where one 

 knows by inference that motility is dominant to absence of motility). 

 But the delay in the first appearance of streptomycin-resistant cells is 

 greater than the delay in the appearance of the first motile cells. 



Hayes: Do you know anything about the actual number of generations ? 



Stacker: In the motility case the delay is of the order of two and a half 

 generation times. I don't recollect the figure for streptomycin. 



Hotchkiss: In the DNA transfer of streptomycin resistance, the situ- 

 ation is approximately as follows. There is a delay of about one division 



