RADIATION AND THE CELLULAR SYNTHESIS OF PROTEIN 79 



we have, as before 



(2) 



We first correct S for the secondary radiation off the path, using the 

 method given by Pollard and BaiTett (1959) and then, iftis the effective 

 thickness and i the number of primary ionizations per cm path of the 

 particles, 



S=So{l-e-it) (3) 



With sufficient variety in the linear energy transfer, which deter- 

 mines i, we can estimate both So and t. These figures are then useful to 

 examine the character of the cell organelle responsible for the effect 

 observed. 



RADIATION ACTION ON AMINO ACID UPTAKE 

 Bacterial cells were grown on minimal medium, using phosphate for 

 buffer, and 5 grams of glucose per litre. At a concentration of 4 x 10^ 

 cells per ml samples were taken and irradiated either in a cobalt source, 

 or in a cyclotron. For cobalt radiation they were irradiated in screw-top 

 culture tubes. Some radiation was done in the frozen state at — 80°C. 

 For cyclotron radiation the cells were placed on fine grain filters backed 

 by a porous layer soaked in minimal medium. The temi)erature during 

 radiation was 2°C. 



After radiation the samples, including unirradiated controls, were 

 incubated in minimal medium with addition of the particular labelled 

 substance under study. At two minute intervals 2 ml samples were 

 withdrawn and either filtered at once on a bacterial filter (collodion 

 membrane, average pore size 0-85 jli), constituting the "whole cell" 

 fraction, or allowed to stand in cold trichloroacetic acid (TCA) for an 

 hour at 2°C before filtering. The whole cell fraction was washed with 

 minimal medium, the TCA insoluble fraction with 5 per cent TCA. The 

 filters were then dried and counted under a thin window counter. 



The results for one case, arginine, are shown in Fig. 1. The five 

 graphs show the uptake in the whole cell (upper gi-aph) and TCA in- 

 soluble fractions. It can be seen that the normal behaviour is a rapid 

 uptake in which all the radioactivity is rather quickly incorporated. 

 The difference between the whole cell and TCA insoluble fraction, 

 designated as "pool", is small. This is in agreement with the findings of 

 Roberts et al. (1957). For small doses of radiation there is no readily 

 detectable effect at all. At 192,000 r the whole cell uptake rises nearly 

 normally and then falls, probably because the cell membrane develops 



