634 D. B. COWIE 
on the yeast Candida utilis growing exponentially in C medium* containing fructose 
as the carbon source. 
Distribution of cellular carbon 
It is extremely important to determine the steady-state distribution of the cellular 
carbon among the chemical fractions of the cell. 
For these studies the yeast cells were grown for many generations in C medium 
containing {!C]fructose as the carbon and energy source. Fig. 1 shows the distri- 
bution of carbon (and phosphorus) among the chemical fractions of the cell during 
exponential growth. Samples of the culture suspension were removed at intervals, 
the cells washed, and chemically fractionated by a modification of the SCHNEIDER 
2 
I 
Total radioactivity in cells 
yl o Tis 











200.0 




















| T 
T 

Cold TCA 
= 
| 
| 
| 
Radioactivity in counts per second 
Soluble 


Fig. 2. Transfer of 4#C from the metabolic pool 
(TCA-soluble fraction) to the protein fraction of 
the cell. Exponentially growing cells were briefly 
| immersed in medium containing [14C|fructose and 
2) 40 80 120 160 200 240 transferred after washing to non-radioactive me- 
Time in minutes dium. 


| 
| 
| 
| 
—- 
| 
| 

method?. The rate of formation of each fraction is shown to be directly proportional 
to the growth rate. Approx. 50°%, of the total carbon is found in the proteins, 810% 
in nucleic acids (one-third of the hot trichloroacetic acid fraction), 10°% in the lipids 
and 12-14%, is always present as “free amino acids”, easily extractable with hot 
water or 5°, trichloroacetic acid (TCA). These “free amino acids” have been shown 
to be contained in a necessary and essential metabolic pool, on the main line of 
synthetic events leading to the formation of protein®+*. 
Specific labeling of the amino acid pool 
Several minutes’ immersion of rapidly growing cells in C medium containing [“C]- 
fructose results in the specific labeling of these pool amino acids. Under these con- 

* C medium: 2g NH,Cl, 6g Na,HPO,, 3 g NaCl, 0.o1 g Mg as MgCl., 3 g KH,PO,, 0.026 g 
S as Na,SO,, too ml 10% maltose and goo ml distilled H,O. 
References p. 645 
