592 Jj. Df. HOLDEN 
sarily retention) of amino acids proceeds effectively only when the energy delivery 
systems of the cell are functioning. The manner in which these activities are coupled 
is completely unknown, although it is easily seen how the ATPase or activation 
systems cited above could consume high energy phosphate compounds during 
transport. How the activities of these enzymes could lead to movement of molecules 
through a membrane is somewhat less certain. The reader is referred to previously 
cited reviews for numerous intriguing speculations on the possible mode of operation 
of membrane catalysts. My reluctance to repeat once again or add to these well 
publicized hypothetical proposals reflects a conviction that the most urgent need at 
present is for more data, some of it of a rather basic nature, as, for example, on the 
composition of the protoplast membrane. It can be hoped that rapid progress of 
studies with mutants defective in amino acid accumulation will delineate the pos- 
sibilities for coupling of transport and energy-yielding systems. 
CONCLUSIONS 
Many microorganisms have been observed to accumulate amino acids in opposition 
to apparent high concentration gradients using an energy-dependent process which 
generally requires that the amino acids react with at least one cell component having 
varying degrees of structural specificity. Extensive evidence for the occurrence of a 
permeability barrier in bacteria and the demonstration of apparent osmotic activity 
of intracellular solutes suggests that amino acids within many microorganisms may 
be largely unassociated with binding sites and thus that accumulation in such cells 
occurs by active transport. Direct, incontrovertible evidence that a major portion 
of the amino acid pool accumulated within microorganisms is in a free state has not 
yet been obtained and, consequently, a final decision that this mechanism alone 
accounts for accumulation in these organisms must still be regarded as being pre- 
mature. Recent studies on mutants deficient in accumulation activity for various 
groups of amino acids offer great promise of resolving this difficult problem. 
ACKNOWLEDGEMENT 
Investigations of the author described here were supported by Grant E-1487 from the 
U.S. Public Health Service and by Contract No. NONR-2702 (00) between this 
institution and the Office of Naval Research. 
REFERENCES 
. ABRAMS, J. Biol. Chem., 234 (1959) 383. 
. ABRAMS, J. Biol. Chem., 235 (1960) 1281. 
. ABRAMS, G. YAMASAKI, J. REDMAN AND C. G. MACKENZIE, Federation Proc., 19 (1960) 129. 
. ANAND AND B. D. Davis, Nature, 185 (1960) 22. 
5 roy ASHWELL, in S. P. CoLtowicK anp N. O. Kapian, Methods in Enzymology, Academic Press, 
New York, 1957, Vol. 3, p. 75. 
6 Y. Avi-Dor, M. Kuczynsk1, G. SCHATZBERG AND J. MAGER, J. Gen. Microbiol., 14 (1956) 76. 
7 J. A. Borzzi AND R. D. DE Moss, Biochim. Biophys. Acta, 49 (1961) 471. 
8 P. D. Bover anp M. P. StuLBERG, Proc. Natl. Acad. Sci., 44 (1958) 92 
9 EK. M. Britt aND P. GERHARDT, J. Bacteriol., 76 (1958) 281. 
10 EF. M. Britt anD P. GERHARDT, J. Bacteriol., 76 (1958) 288. 
11 R. J. BRITTEN, Science, 124 (1956) 935. 
12 R. J. BRITTEN AND F. T. McCiureE, Bacteriol. Revs., (1962) In press. 
ag ee 
