314 H. B. ROBERTS. R. J. BRITTEN. AND B. J. MCCARTHY 



The maxinium entry flow (a) is equal to the flow (d) which is 

 required for the synthesis of nucleic acid (NA). Furthermore, the flow 

 due to endogenous synthesis (e) is regulated by feedback control so that 

 a -\- e = d. Thus the pool (S) remains constant and cannot be expanded 

 or depleted. The pool (Pi cannot contain more than 5 seconds' supply of 

 uracil compounds or an observable kinetic delay would appear in the 

 entr>' of C'^-uracil into NA. Forty per cent of the exogenous uracil 

 proceeds directly to NA and 60% enters the pool S, so that the flow 

 (b) is roughly 607c of (a). 



Labeled bases, once incorporated into the pool, are not rapidly 

 removed by exchange with external material. Hence the incorporation of 

 tracers into nucleic acid cannot be abruptly terminated by dilution of 

 the exogenous tracer material. The best condition which can be achieved 

 is the abrupt transition from phase 1 to phase 2. By allowing only a very 

 short time before the external material is exhausted, a ratio of 20: 1 in 

 the rates of phase 1 and phase 2 can be obtained. 



The conversion of uracil to cytosine causes a further complication 

 in the kinetics of incorporation because the proportion of the labeled 

 material which enters nucleic acid as cytosine increases with time. 



Guanine, adenine, and cytosine show a similar behavior but the pool 

 sizes and the proportions entering directly differ, as shown in Table V. 



TABLE V 

 Incorporation Ch.\r.\cteristics of Four RNA Bases 



Direct entry Pool (S) time constant 



Base C^)" (minutes)'" 



Uracil 40 + 5 10+2 



Cj'tosine 45+5 28 + 5 



Guanine 78 ± 5 3.3 + 0.5 



Adenine >50 3-9<^ 



" Shows the initial rate of incorporation of the tracer into RNA as per cent of the 

 ultimate rate, per unit cell mass. 



* Shows the time constant (1/e) of the pool (S) derived from the exponential decay 

 of its radioacti\ity during the second phase of an experiment at low concentration such 

 as Fig. 11. These values are in agreement within the errors indicated with the values 

 derived from the time const-ant with which the incorporation into RXA (at high con- 

 centrations such as Fig. 12) approaches its ultimate rate. 



« The pool of adenine compounds does not show a single time constant and a detailed 

 analysis to resolve the individual time constants has not been carried out. 



Once the complications introduced by the pool are recognized and 

 measured, appropriate corrections to the kinetics obser^-ed in the macro- 

 molecular components can be made. Without this understanding, the two 



