AMINO ACID POOL FORMATION IN Escherichia coli 605 

As the pool rises, the reverse reaction (AR + E + AE + R) reduces the quantity 
of free carrier (E). Thus, the rate of formation of carrier complex with free amino 
acid falls until it equals the rate required for protein synthesis. This is then the steady 
state. 
Referring again to Table I, the deductions from this model will now be compared 
with the observations. Item I is obvious, since the energy requirement has been 
built in. Two distinct properties of the model lead to maintenance of the pool when 
formation is suppressed (items 2, 3, 4 and 5). We could assume the reverse reaction 
(AR -- E-+ AE + R) to be energy-dependent, and then clearly the pool would be 
mai tained in the absence of glucose. Alternatively, we are at liberty to choose a 
small value for the constant k, without influencing any other properties of the model, 
and therefore the loss rate under all conditions (short of damaging the cell) can be 
set as low as necessary. The choice of a small ky simply means that the carrier (or 
catalytic site) has a high affinity for the amino acid, and thus the amount of carrier 
complex will saturate at low concentrations. This is consistent with the saturation 
of the exchange rate (item 14) and of the formation rate (item 7) at low external 
concentrations. 
The evidence on exchange between pool and external amino acids led to the postula- 
tion of the exchange processes described in items d and e in Fig. 8. These processes 
are sufficient to explain all the observations on exchange (items II, I2, 13, 14 and 
15 of Table I). Saturation of the exchange rate at low external concentrations is 
predicted if the natural assumption is made that the exchange rate between free 
amino acid and carrier is more rapid than the exchange rate between the amino 
acids of the carrier complex and the site complex. Thus, when exchange is studied 
with labeled amino acid, the specific radioactivity of the amino acid associated with 
the carrier complex would always be close to that of the external amino acid. The 
amount of the carrier complex is saturated under the conditions of the experiment 
at o°. The collisions between the carrier complexes (constant specific activity and 
quantity) and the site complexes would control the exchange. The rate of exchange 
would therefore be independent of the external concentration and proportional to 
the pool size. 
In the mathematical appendix of ref. 12, the carrier model is examined in some 
detail. Allowance has been made for two different kinds of sites. In this model it is 
important to account for the utilization of amino acid for protein synthesis, the 
native pool and for the competition between internally synthesized and exogenous 
amino acid if quantitative evaluation of the model is to be made. The constants of 
the model (for proline in EF. colz) which give a relatively good quantitative correla- 
tion with the data are presented in the appendix!. These constants were obtained 
from experiments on pool sizes and rates of formation. It is very pleasing that so 
much information on the details of the concentration relationships and the compe- 
tition between synthetic processes and concentrating processes can be encompassed 
in one conceptually simple model. One interesting consequence of the treatment is 
that the two sets of sites correspond to two components of the pool of quite different 
binding, one of which has a saturation value 20 times the other. The larger component 
has a half-saturation value of external concentration which is roo times that of the 
smaller and, as a consequence is only dominant at the higher end of the concentration 
range. It is very satisfying that the existence of two components of the pool of such 
References p. 609 
