444 NITROGExN METABOLISIM 



Furthermore, every kind of amino acid in the peptide chain recurs at 

 constant intervals. For example in silk fibroin (a protein) they find that 

 every other amino acid residue is glycine, every fourth one is alanine, and 

 every sixteenth one is tyrosine. This arrangement can be diagrammed as 

 follows, X representing other amino acid residues than those mentioned : 



G-A-G-X-G-A-G-T-G-A-G-X-G-A-G-X-G-A-G-X-G-A-G-T-G-A-G-X 



A similar recurrence of amino acid residues in a periodic manner was 

 found in the molecules of other proteins. The various kinds of proteins differ 

 from each other in the kinds and frequencies of the constituent amino acids. 

 If the results of these workers regarding the orderliness of the arrangement 

 of amino acid residues in protein molecules is confirmed as a general principle, 

 an important step will have been taken in elucidating the intricate problem 

 of the structure of protein molecules. 



The principal regions of protein synthesis in plants do not necessarily 

 correspond to the principal regions of amino acid synthesis. In fact a clear 

 distinction should be made between these two processes. In some species of 

 plants most amino acid synthesis occurs in the roots. In all plants most 

 condensation of amino acids to proteins occurs in meristems or in storage 

 tissues, although some protein synthesis can probably occur in most cells. 

 Amino acids are often translocated from the tissues in which they originate 

 to other, often distant, tissues before being condensed to proteins. It is 

 generally believed that little or no translocation of proteins as such occurs in 

 plants. 



In meristems condensation of amino acids results principally in the con- 

 struction of protoplasmic proteins. This is one phase of the process of assimi- 

 lation (Chap. XXXI). In many seeds and some other organs condensation 

 of amino acids results in the production of storage proteins. This is a phase 

 of the process of accumulation (Chap. XXXI). Most such proteins are subse- 

 quently digested to amino acids which, usually after translocation to other 

 tissues, are assimilated. Reutilization of assimilated proteins may also occur. 

 In senescent leaves, for example, decomposition of protoplasmic proteins may 

 take place and some of the resulting organic nitrogenous compounds, at least, 

 may be translocated to meristems and there resynthesized into proteins. The 

 condensation of amino acids in the formation of proteins is probably catalyzed 

 by enzymes of the proteinase group (Chap. XXVII). 



The nucleoproteins are a group of complex proteins which constitute 

 a large proportion of the proteins in the nuclei of both plant and animal 

 cells. The chromosomes are apparently composed chiefly of nucleoproteins, 



