FREE NITROGEN COMPOUNDS IN PLANTS 689 
The labeled products of hydrolysis were then supplied to the petiole of similar bean 
leaves, and their reincorporation in leaf protein studied. More radioactivity was 
found in the synthesized leaf protein when the partial hydrolyzate was supplied than 
when the free amino acids of complete hydrolysis were furnished. From this it could be 
inferred that peptides were preferred intermediates in the synthesis of this protein. 
However, there is the possibility that the partially hydrolyzed proteins may have 
entered the plant cells more easily than the completely hydrolyzed, and would thus 
be more readily available for protein synthesis even if they had first to be converted 
to their constituent amino acids before being incorporated into protein. 
It is still a very puzzling feature that one of the best documented cases of peptide 
bond formation in plants is that of HANEs ef al.44, ©, which, however, leads to y- 
glutamyl bonds which are not the kind required to build protein directly. It is sug- 
gestive that these y-glutamyl bonds involve glutamine which so often seems to play 
a key role in protein synthesis in plants, and that there is also a mechanism for 
incorporating energy from ATP into this y-linkage. It was suggested earlier (cf. ref. 
49, p. 381) that the protein-synthesizing surface might well act first as a y-glutamyl 
peptidase, and in this way the y-glutamyl peptide could be regarded as a form in 
which the nitrogen for synthesis and the energy in the y-linkage could be presented 
acceptably to the synthesizing surface. On this view, glutamine would in effect 
become an amino acid carrier; this would be consistent with its ubiquitous occurrence 
in active centers of protein synthesis in plants. It may also be mentioned that in the 
work of WAELSCH® glutamine often acts as a “starter” of protein synthesis in bac- 
terial cultures. An interesting development along these lines is that, as communicated 
to this conference, THOMPSON (cf. this Symposium, p. 54) now finds it possible to 
detect in plants a variety of y-glutamyl peptides, some of which correspond to the 
protein amino acids. 
All the foregoing discussion indicates, therefore, that the immediate precursors of 
protein in plants are amino acids in some form or another, but it is equally evident 
that the amino acids which are immediately involved in the synthesis are certainly not 
those which constitute the often large stores of free soluble compounds. These amino 
acids which occur in bulk are in fact often in different compartments in the cell. 
Moreover, the composition of these soluble stores never reflects the amino acid com- 
position of the resultant protein®*’. The problem, therefore, is to identify correctly 
the sites of both amino acid synthesis and of their incorporation into protein. 
The interpretation of the carrot tissue culture sytem was that this site of protein 
synthesis was remote from the stored amino acids, was more accessible to sugar as a 
source of carbon than to the stored amino acids and, if amino acids were separately 
formed prior to their entry into the protein as they must surely be, then these indivi- 
dual molecules were not free to mingle with the amino acids of the soluble pool*: 4, 
YEMM AND FOLKES” have erroneously interpreted this scheme to mean that amino 
acids are not intermediates of protein synthesis. This was neither implied nor implicit 
in the scheme, nor is it necessary to make such a restriction until both the site and 
the mechanism of the primary synthesis of protein in the plant cell are precisely 
known. What is clear is that the amino acids that participate intimately in the 
actual formation of protein at the site of synthesis may be, and apparently are, 
formed there, and they do not mingle freely with those that are stored in bulk in 
the soluble phases of the cell. 
References p. 692/693 
