312 



H. H. HOHKKIS, H. .1. HHII'IKN . AND H. J. MCCARTHY 



in formation can be derived indirectly from simple measurements. The 

 rate of protein synthesis per unit mass of cells is constant; thus, the 

 total rate of entry of radioactivity into proteins of all types must measure 

 the specific radioactivity of the precursor. Direct measurements of the 

 pool and product jirotein have shown that the rate of entry into the 

 protein was proportional to the specific radioactivity of the pool. This 

 observation indicated that the amino acid pool was in fact a precursor 

 to protein (Britten et al., 1955). 



The rapid establishment of the full rate of entry of S^^04"" into 

 protein indicated that there was no appreciable pool of sulfur compounds 

 (McQuillen et a/., 1959). Leucine also shows prompt entry into protein 

 (Britten et al., 1962) but a mixture of amino acids from a protein 

 hydrolyzate may show a lag before the full rate is established. This is 

 presumably due to the existence of quite large pools of glutamic acid, 

 alanine, and aspartic acid. 



P^-04 is far from an ideal tracer because of the large pool of phos- 

 phate and phosphorylatcd compounds. There is a further delay intro- 

 duced by the time required for the P''" to reach the a position of tiic 

 nucleotides (Bolton and Roberts, 1956). In E. roll the specific radioac- 



3CXX) 



Uptake of lO'^M C'"- uracil at 37°C 

 f. Co// ML- 30, 1 mg (wet) /ml 



2000 



% 



.£ 1000 



I 



Time in minutes 



Fig. 11. Incorporation of C"-uraciI into the metabolic pool and the HXA of 

 E. coli. The difTerence between the two curves measures the radioactivity of the 

 pool. The initial concentration was low; 10"' Af. 



tivity of the nucleic acid precursors rises roughly as (1 — e~*/^") where 

 Tq is 600 seconds. This slowly rising temi makes it very difficult to 

 observe the kinetic delays introduced by subsequent smaller pools of 

 ribosome precursors. 



The nucleic acid bases provide much more suitable tracers for kinetic 



