II UTILIZATION OF PROTEIN PRECURSORS 5O7 



hemoglobin (Muir, Neuberger and Perrone, 1952), various muscle proteins (Simp- 

 son and Velick, 1954), and milk proteins (Askonas, Campbell and Work, 1954), 

 isolated after the injection of labelled amino acids into the intact animal, gave no 

 indication of an unequal distribution of the tagged amino acid in the peptides 

 derived from these proteins. Whether these two sets of results actually indicate 

 the operation of two different mechanisms has been questioned by Steinberg et al. 

 (1956). He points ovit that for reasons of kinetics unequal rates of incorporation 

 of labelled amino acids into "intermediate conjugates" would be maintained 

 only for a short time. Correspondingly the unequal labelling of the parts of the 

 protein molecule which are made up from these conjugates would be detectable 

 only during a limited time span after addition of the tracer. However, Borsook 

 (ig56a, pp. 62-65) contends that non-uniform labelling could be explained as well 

 by a rapid exchange of amino acids after adsorption on the "template", but prior 

 to peptide link formation. The investigation of this problem in embryos may prove 

 to be laborious and difficult since it presupposes work with substantial amounts of 

 highly purified proteins. However, exploration of this mechanism in proliferating 

 and in differentiating tissues may provide a comparative approach of considerable 

 interest both for the embryologist and the protein chemist. 



III. THE PROTEIN FORMING SYSTEM (PFS) OF DIFFERENTIATED AND 



EMBRYONIC CELLS 



In the preceding section the PFS was discussed from the standpoint of the rate 

 and the possible mechanisms of the utilization of protein precursors. For an under- 

 standing of the changes in the utilization of protein precursors and the production 

 of new types of cell proteins, which occur during development, a more direct 

 examination of the components of the PFS and of their mode of operation at differ- 

 ent stages of development, is required. As yet, too little knowledge is available 

 about the PFS in the developing cell to even consider such an account. Most of 

 the work carried out so far with embryonic tissues has followed such divergent 

 methodological approaches which were applied under such different conditions 

 that no coherent picture can be given of the changes of even a single parameter 

 of the PFS throughout the main phases of development. Incorporation studies 

 with microsomes in sea urchin eggs, the use of purine analogs combined with 

 radioautography in the chick embryo, and measurements of uptake of ^^P in 

 amphibian development have yielded so far only a loose patchwork of obser- 

 vations concerning the PFS in embryonic cells. However, considerable advances 

 have been made during the past few years in elucidating the nature of the PFS 

 of adult cells, and it may be most profitable to use the data obtained from differ- 

 entiated animal and microbial cells to indicate some fruitful approaches to an 

 analysis of the PFS in developing tissues. 



Decisive progress in the analysis of protein synthesis has resulted essentially 

 from two lines of research on differentiated cells. The one deals with measurements 



Liierature p. 53g 



