E. W. YEMM 



are exchanging with the protein. Is there any evidence that this is so, and if so which 

 amino-acids are concerned? 



E. W. Temm. The distribution of 15 N in the proteins of barley roots has not been 

 examined in detail. However, with leaf-tissue proteins we have evidence that the 

 isotope is incorporated to the greatest extent in glutamic, aspartic and amide nitrogen 

 of the protein, although there are appreciable amounts in the monocarboxylic and 

 basic amino-acids. From this and other work it seems probable that the abundance 

 of 15 N in the different amino-acids of the tissue proteins reflects the extent to which 

 the amino-acid becomes labelled in the metabolic pool. In both plant and animal 

 tissues, supplied with isotopic ammonia, incorporation is usually greatest in glutamic, 

 aspartic and amide nitrogen, probably owing to the ease with which these are 

 synthesized from ammonia. 



A. J. Willis. The incorporation of 15 N into the protein of barley roots is much more 

 extensive in the amide groups than in the total nitrogen of the protein. This indicates 

 extensive exchange reactions involving these amide groups. 



J. Bracket. In connexion with Dr. Yemm's suggestion that there might be two differ- 

 ent mechanisms involved in protein synthesis (transpeptidation and template 

 activity), it is worth pointing out that Koritz and Chantrenne recently obtained 

 evidence for such a viewpoint: in reticulocytes, incorporation of labelled amino- 

 acids precedes the peak in RNA synthesis; this peak coincides with the formation of 

 various enzymes, which might be produced by a specific template mechanism. 



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