VII. SYNTHESIS OF RNA AND RIBOSOMES 



313 



studies of RNA synthesis. In sinte of the presence of i)ools of nucleoside 

 di- and triphosphates, exogenous purines and pyrimidines are rapidly 

 incorporated into nucleic acid (Bolton and Roberts, 1956). Recent studies 

 of incorporation of C'^-uracil (McCarthy and Britten, 1962) and other 

 bases (Buclnvald and Britten, 1962) have given a much more detailed 

 understanding of the process. 



Figures 11 and 12 show two characteristic features of the incorpora- 

 tion process. At low concentration two phases of the process are evident, 

 one a rapid rate of incorporation into nucleic acid which persists as long 

 as external uracil is present, followed by a much reduced rate which 

 occurs when the uracil is utilized solely from the pool. At higher concen- 



20 



30 



40 



Time in minutes 



Fig. 12. Incorporation of C'Miracil into the RNA of E. coli from high concentra- 

 tions of external uracil. The ordinate scales of the two curves are different. 



trations the two phases are also observable. However, in this case the 

 rate of incorporation during the second phase is more rapid, as the direct 

 incorporation continues throughout and is supplemented during the 

 second phase by C"-uracil drawn from the pool. 



To account for the direct entry of C"-uracil together with utilization 

 of pool material a mechanism such as that shown in the diagram below 

 must be envisaged. 



Exogenous 

 C"-uracil 



Endogenous 

 C'--uracil 



/• 



NA 



b\\c 



