KADIATIOX AND THE CELLULAR SYNTHESLS OF TKoTKIX 



83 



these could be rejDreseiited by one or other of the various classes of 

 ribosoiue. It is possible that methionine also fits into this class. In any 

 event, the sensitivity of methionine uptake markedly differs from the 

 other amino acids, suggesting that some other mechanism is involved 

 for the incorporation of methionine. 



The data for the other 5 amino acids agree with a sensitive region 

 having a molecular weight of between 3-5 and 5 x 10^ and having one 

 thin dimension, roughly estimated as 30 A thickness. The molecular 

 weight agrees rather well with that of the ribosomes, being equivalent 

 to a particle of 70 Svedberg units. The shape does not agree and it is 

 suggested that for these 5 amino acids the operating condition is one 

 which is unfolded, while in preparations in broken cells, the particle 

 is rolled up or folded in some way. Perhaps it is the function of the 

 energy source to provide the unfolding. This w^ork was done in collabora- 

 tion with Dr. E. S. Kempner. 



RADIATION ACTION ON THE FORMATION OF AN ENZYME 

 The previous study is concerned with the almost immediate incor- 

 poration of a labelled amino acid in the presence of a minimal medium. 

 The radioactive tracer method is very sensitive and the studies are 



30 



60 90 120 ISO 



Time (mm) 



180 210 



Fig. 5. — The effect of various doses on the time course of production of /3-galactosidase 

 in cells already induced. For low doses there is a small increase initially. In general the 



enzyme is depressed and delayed. 



relatively simple. If we turn to the formation of an actual enzyme, 

 ^-galactosidase, the sensitivity of assay is harder and it is not so easy to 

 detect differences at short times. Using cells already induced by growth 



