POLLARD AND KENNEDY 

 100 



143 



30- 



METHIONINE 



\ 



■ l 



OPTICAL DENSITY 



\ 



COLONIES 



\ 

 \ SULFATE 



PHOSPHATE \ 



■~" 50 lOOxlOOOr 



RAOIATION DOSE 



Fig. 6. Logarithmic dose response plots for colony counts, optical density, and phos- 

 phate, sulfate, sulfide, and methionine uptakes. The relative sensitivities can be analyzed 

 in terms of sensitive volumes, which are given in table 1. 



The most surprising result is the great sensitivity of phosphorus uptake. It 

 is difficult to see how the relatively simple processes involving phosphate can 

 be involved with such large target units. A repeat of the uptake, using C mini- 

 mal medium, has been made, with results still indicating a great sensitivity, 

 though perhaps not as great as in Tris. We are at a loss to explain the finding, 

 and rather than speculate about the control of phosphorus uptake by large units 

 of DNA we prefer to continue studies aimed at finding the nature of the dam- 

 aged products. It should be pointed out that, with all three tracers, phosphate, 



