AMINO ACID TRANSPORT IN MICROORGANISMS 587 
quent distention of its membrane would be inhibited by the deposition of additional 
wall substance. Since only one procedure (sucrose supplementation) would be ex- 
pected to reduce significantly the influx of water to intracellular sites, whereas 
both stimulatory conditions have in common the prospect that membrane distention 
would be inhibited, it can be concluded that the latter effect is decisive and that 
the membrane plays an active role in the accumulation phenomenon. Cell membrane 
distention could reduce accumulation capacity by allowing an equilibration of influx 
and efflux rates at a lower than normal intracellular concentration. Since the rate 
of accumulation is the same no matter when the labeled amino acid is added to 
glycolizing cells, it is likely that any distention which does occur in unprotected 
TABLE VI 
ACETATE INCORPORATION INTO CELL WALL OF L. avabinosus 
LB, cells HB, cells 
Addition to buffer = = a 
(counts|min|ro mg cells) * 

[2-14C] Acetate IQ 700 38 000 
2-4C]Acetate + NH,+ + B, + t-glutamic acid 44 600 45 000 

* Incubation was for 90 min at 37° using conditions and extraction procedures described in 
Table V. Supplements to the incubation buffer were provided at the following concentrations: 
NH,Cl (0.0033 M), pyridoxamine, 2HCI (0.084 ug/ml), and L-glutamic acid (0.003 ). 
vitamin Bg-deficient cells is associated with the accumulation of amino acid and, 
therefore, that the latter is osmotically active. With the exception of the protoplast 
swelling studies cited previously, efforts to obtain more direct evidence for the 
existence of accumulated amino acids in an osmotically active form have been 
inconclusive. 
Additional support for this interpretation of our findings is that these effects are 
not confined to vitamin B,-deficient cells. Biotin- and pantothenate-deficient cells as 
well as those harvested in the early exponential phase of growth from nutritionally- 
adequate media all accumulate amino acids in an abnormal manner?’. Normal 
accumulation is restored in biotin-deficient cells either by high concentrations of 
sucrose (Fig. 10) or by osmotically inconsequential levels of acetate (Fig. 11)**. 
Comparable responses are observed using acetate and pantothenate or sucrose with 
pantothenate acid-deficient cells and acetate alone or sucrose alone with nutritrionally 
normal early exponential phase cells. All such cell types contain smaller amounts of 
cell wall material than do the late exponential phase cells normally used as controls 
in such studies. It appears then that any procedure which reduces the quantity of 
cell wall also reduces the accumulation capacity (without reducing the initial accumu- 
lation rate). Conversely when conditions are provided which permit increases in 
wall mass there is a concomitant expansion in the amino acid accumulation capacity. 
This is true, of course, only in cells in which accumulation capacity is submaximal 
to begin with. 
From these and the previously cited studies it seems highly likely that a number of 
References p. 592/594 
