678 F. C. STEWARD AND R. G. S. BIDWELL 
period there was an increase in the total quantity of the protein but only a relatively 
slight increase in its radioactivity, although the soluble components were still heavily 
labeled. This result shows, therefore, that the newly synthesized protein must have 
derived its carbon from an unlabeled source immediately accessible in the products 
of photosynthesis and not from either the preformed and stored sugar or from the 
amino acids in the cell, which were heavily labeled. The data which support these 
conclusions are given in Table IV. 
TABLE IV 
SOURCE OF CARBON FOR PROTEIN SYNTHESIS IN CORN 
Data of HELLEBUST AND BIDWELL, unpublished. Corn leaves were supplied with “CO, 
for 5h, then #2CO, for 64 h. Corn root tips were supplied with [Cjglucose for 2 h, 
then [!2C]glucose for 7 h. 


Corn leaves Oe 
Time after supply of C (h) 24-64 2-7 
Average specific activity of soluble sucrose (counts/min/wg carbon*) 75 240 
Average specific activity of soluble hexose (counts/min/uwg carbon*) 75 48 
Average specific activity of soluble amino acids (counts/min/uwg carbon*) 144 213 
Average specific activity of protein amino acids (counts/min/uwg carbon*) 84 2 
Increase in carbon of protein amino acids (wg carbon) 35 3.0 
Increase in activity of protein amino acids (counts/min) 280 243 
Carbon entering protein: if from soluble sugars (wg carbon) 3.8 1.0 
if from soluble amino acids (ug carbon) 1.9 iLgli 
Protein synthesized from non-active source (%) 90-95 66 


* These values did not change much during the experimental period; hence the average values 
are sufficiently accurate for the calculations. 
Similar conclusions follow from experiments on excised roots of corn which re- 
ceived exogenous supplies of [!C]sugar followed, in an ensuing period, by non- 
radioactive sugar. These data are also cited in Table IV. 
Thus the overall conclusion is that the ability of a given compound to furnish 
carbon for protein is not only determined by its nature but by its location within 
the cell. 
Another example flows from the work of RAcussEN AND Hosson*’. This work 
shows that carbon supplied exogenously as @CO,, like the endogenous sugar in the 
carrot explants, is much more readily accessible to the site of protein synthesis. Thus 
the long-known efficiency of the green leaf in the synthesis of protein is seen to be 
due to the ready availability at the site of synthesis of newly made photosynthetic 
products. RACUSSEN AND Hopson showed the prompt conversion of the C-containing 
sugar to [!@C]aspartic and [!4C]glutamic acid in the protein and the even more 
prompt use of the CO, for the formation of arginine and lysine in the protein. In 
other words, the accessibility of carbon for protein synthesis is much greater in this 
system at the site where CO, is being reduced. 
It has already been mentioned that BAssHAm et al. have arrived at similar con- 
clusions. These workers now believe that the “C which enters protein of chlorella 
References p. 692/693 
