98 



S. S. COHEN 



Most recently, it has been possible to study extensive protein synthesis in 

 a particular strain of E. coli requiring both thymine and uracil for growth 

 (Earner and Cohen, 1958). In the absence of thymine, DNA is not made; net 

 synthesis of RNA does not occur in the absence of exogenous uracil. In a 

 medium devoid of both pyrimidines a net synthesis of nucleic acid does not 

 occur, but such organisms make protein at a normal rate for considerable 

 periods (as in Fig. 14) and may even be induced to make a new enzyme, 



390 



-350 



-310 



-270 p 



-230 q 



Fig. 14. The synthesis of protein in a thymineless, uracilless strain of E. coli under 

 different conditions of pyrimidine deficiency (Bamer and Cohen, 1958, in press). 



xylose isomerase. On analysis of these cells synthesizing protein in glucose- 

 C^^ in the absence of exogenous pyrimidines, it was observed that a fraction 

 of the RNA (up to 15 % of the total) was turning over, since RNA could be 

 isolated coiistaining uridine m which preformed uracil was now Hnked to 

 ribose formed from the isotopic glucose. 



In summary, we may say that, at the present time, no case (with the ex- 

 ceptions noted in footnote 1. p. 54) is known in which protein synthesis has 



