688 F. C. STEWARD AND R. G. S. BIDWELL 
of the protein rests on measurements of ATPase activity or upon the incorporation 
of 4C from amino acids into non-particulate protein. When the preparations are 
supplied with !C-labeled amino acids, complexed with what is described as “polynu- 
cleotide (SRNA)”, both the radioactivity of the total protein and the enzymatic 
activity toward ATP were increased by a factor of 2.7; this increase was compared 
with the effect observed when the free amino acids were supplied alone. From this it 
is concluded that the nucleotide—amino acid complexes are more effective donors into 
this specifically identifiable protein than the free amino acids alone. In other words, 
this system which requires ATP, GTP and RNA constitutes the amino acid-activating 
system of protein synthesis. But some alternative suggestions for plants have also 
been made, or can be adduced, and these will now be mentioned. 
In Torulopsis COWIE AND WALTON!?? now visualize an amino acid pool from which 
protein is constituted. However, the pool amino acids are not regarded as those 
which are free in the cell, but they are associated with macromolecules. Thus the 

Exogenous Endogenous 
arginine glutamic Dees ; 
| Empty site Lit reset 
Cell 1 —Cold TCA soluble fraction —+|precipitable| 
wall | | fraction | 
Fig. 3. Model to explain behavior of glutamic family of amino acids (From CowlE AND WALTON!®). 
binding of these amino acids is really equivalent to segregating them in a separate 
compartment. The normal source of the pool amino acids is by synthesis from sugar 
via the Krebs’ cycle, and once an amino acid becomes part of the pool its conversion 
to protein is not interfered with by the competitive supply of exogenous amino acids. 
However, in occupying the binding sites, exogenous and endogenous amino acids 
may compete. This scheme (illustrated in Fig. 3) differs from that which was elabo- 
rated for carrot cells, principally because it does not include any concept of protein 
turnover, or of the independent metabolism of protein breakdown products in the 
phase or compartment which is associated with the storage of the soluble nitrogen 
compounds. This difference, however, may well be intelligible in terms of the different 
morphology and organization of the two systems. In yeast, more of the reactions 
seem to occur on the periphery of the cell, metabolic products are more readily ex- 
creted to the medium, and there is no analogue for the large aqueous vacuole of the 
parenchyma cell of plants in which so much organic material is stored internally. 
MEDVEDEV” labeled the cytoplasmic proteins with C, extracted them and then 
subjected them to partial or complete hydrolysis with pepsin or HCl respectively. 
References p. 692/693 
