AMINO ACID TRANSPORT IN MICROORGANISMS 569 
as the external amino acid concentration was varied within physiologically reason- 
able limits. A notable example is that of lysine accumulation by S. faecalis described 
in GALE’s initial report?’ which was consequently thought for this and other reasons 
to occur by a passive diffusion process. Lysine accumulation by another strain of 
S. faecalis studied in this laboratory shows saturation kimetics and dependence on 
glucose metabolism®?. The same is true for lysine accumulation by S. cereviseae*. 
A summary of accumulation rates observed in representative studies is presented 
in Table I. In Gram-negative bacteria accumulation is usually completed in approx- 
TABLE I 
RATES OF AMINO ACID ACCUMULATION IN VARIOUS MICROORGANISMS 


Concentration 
Half maximum 
f for half 
vate 

Organism Amino acid pmoles|min| care Ref. Numbers 
LODE pemoles|ml 
L. avabinosus Glutamic acid 0.80 0.50 54 
Alanine itoat 2.0 52 
S. faecalis R Alanine Tie? 0.20 52 
Lysine 1.9 0.50 52 
S. faecalis Glutamic acid 0.45 0.50 27 
E. colt Proline 0.80 0.003 2 
Valine 1.0 0.0005 12 
S. ceveviseae Glutamic acid 4.2 0.30 41 
Arginine 3.0 0.40 41 
Lysine 2.4 0.30 41 

imately one minute at 37° so that accurate rate measurements are difficult to achieve. 
The values shown, therefore, shou!d be regarded as minimal and, in any case, must be 
appraised cautiously since the process is undoubtedly complex and probably involves 
a number of components. There is no certain knowledge of the nature of the rate- 
limiting process. Accumulation rates in Gram-negative and Gram-positive bacteria as 
well as in yeasts appear to be comparable although in the two classes of bacteria 
they are achieved at substantially different external concentrations. 
The capacities normally observed also differ markedly in these groups of organisms. 
Illustrative examples of the maximum capacity and the maximum apparent con- 
centration ratios (apparent internal concentration to external concentration) for a 
variety of amino acids and organisms are presented in Table II. Gram-negative 
organisms generally accumulate amino acids to one-fifth or one-twentieth of the 
level observed in Gram-positive bacteria, but achieve gradients at least as high as 
those attained in Gram-positive organisms. However, BRITTEN, ROBERTS, et al. have 
shown that the size of the proline pool in E. coli is dependent on the osmotic strength 
of the extracellular medium?®; 1, Using proline or a mixture of amino acids pool 
levels as high as 100 zmoles/10o mg have been obtained by incubating cells in sucrose- 
containing media. Exceptionally high apparent gradients appear to exist for en- 
dogenously synthesized amino acids in E. coli (valine 28 000, glutamic acid 7 300)". 
Since this material is present in minute amounts, its state and, therefore, the reality 
of these gradients is open to question. 
References p. 592/594 
