598 R. J. BRITTEN AND F. T. MCCLURE 
3) Peptides do not appear to be intermediates in protein synthesis. 
4) An energy source (such as glucose) is required for pool formation to occur at 
normal rates but is not required for maintenance of the pool for relatively long 
periods. 
5) Specific pool formation mechanisms exist for each amino acid or group of 
structurally similar amino acids. 
6) For any given amino acid there appears to be maximum pool size (or saturation 
value) at large external concentrations. 
7) Any damage to the cells’ integrity, or even mild treatments (for a bacterial 
cell), such as osmotic shock, leads to loss of the pool. 
8) Exchange between pool and external amino acids occurs at a high rate, not only 
when there is steady flow through the pool but also in absence of glucose or at 0° 
when the flow through the pool is strongly suppressed. (Conditions which also 
suppress pool formation. ) 
DISCUSSION OF THE MECHANISM OF POOL FORMATION 
Introduction 
The purpose of this discussion is to examine the implications of the large number 
of experimental observations!. It appears necessary, in order to bring some clarity 
to a problem of this complexity, to start out by postulating models of the process. 
The discussion of the experimental observations in relation to the models allows the 
implications to be brought out more clearly. 
The qualitative predictions of the relatively simple models considered here can 
be deduced easily. However, our knowledge of their quantitative predictions must 
depend on a fairly crude analysis. What are undoubtedly complex reaction sequences 
are taken to be single steps subject to the simpler equations of chemical kinetics. 
The coupling of the concentration process to the cell’s energy supply is, for example, 
included in an almost purely symbolic way. 
10 
Lmoles/g wet wt. 

O 10 20 30 
Time (min) 
Pig. 5. Stability of the proline pool after reduction of the external concentration. EF. coli strain 
15 I~ A~ U~ suspended at 0.5 mg wet wt./ml at 25° in the absence of thymine, arginine and 
uracil. Glucose present at 2 mg/ml. At time zero [4C]proline was added (@) to a concentration 
of 10-° M (20 wmoles/g wet wt.). At 11 min (shown by arrow), part of the culture (+) was 
diluted by a factor of 15 and correspondingly large samples were taken. The concentration of 
(4C\proline after dilution was 5 x 10-7 M. The incorporation into protein is shown by the lower 
set of points (x ). The initial rate of uptake was 1.5 wmoles/g wet wt./min. The rate of loss from 
the pool is not measurable, but certainly less than 1/too of the rate of uptake. 
References p. 609 
