V CONCLUSIONS CONVERGENCE OF CONCEPTS 53 I 



are distinguished by a metabolic lability which leads to their preferential break- 

 down by an unknown mechanism. 



In contrast to yeast, in other microbial forms like E. coli, no indication was found 

 of any degradation of proteins into amino acids and the utilization of these degrada- 

 tion products of labile proteins for new protein synthesis under conditions of rapid 

 growth (Hogness, Cohn and Monod, 1955). However, later experiments with 

 E. coli showed enzyme induction in the absence of growth, suggesting conversion 

 of some bacterial proteins into enzyme proteins (Rickenberg and Lester, 1955; 

 Lovtrup, 1956). Correspondingly, it would be of considerable interest to find out 

 whether in differentiating embryonic tissvies different cellular proteins are broken 

 down at differential rates and to what extent this protein breakdown is compatible 

 with or requisite to the further development of the cell. For example, in the 

 case of the replacement of embryonic erythrocytes into adult red blood cells 

 the fetal hemoglobin is presumably liberated in the spleen after destruction 

 of the embryonic erythroblasts. Whether the fetal hemoglobin has to be com- 

 pletely degraded into amino acids or whether larger peptides can be utilized 

 for the resynthesis of the hemoglobin molecule of the mature organism has 

 not been investigated. Also, it is not known as yet for other proteins such as 

 myosin, lens protein, etc. whether there exist chemically distinct embryonic and 

 mature forms as in the case of hemoglobin ; whether in such instances the conver- 

 sion of the embryonic protein into the adult protein form requires complete 

 degradation to amino acids; and whether formation of the adult form occurs in the 

 same cell or in subsequent cell generations. Possibly, the degradation of yolk 

 proteins to amino acids for utilization in the synthesis of cytoplasmic proteins may 

 provide excellent material for an understanding of the interconversion of cellular 

 proteins in general. 



The similarities of the PFS in embryonic cells, in microbial forms and in mature 

 animal and plant tissues have become apparent from the study of its constituents, 

 in particular of the RNA moiety and of the structural organization of the protein 

 forming system itself. It may be recalled that the first suggestion of an importance 

 of RNA in protein synthesis came from the investigations of Caspersson and of 

 Brachet on rapidly growing, essentially embryonic, tissues. The initially tenuous 

 correlation between RNA and protein formation has become more firmly estab- 

 lished through microbiological studies and through the investigation of protein 

 synthesis in such highly differentiated cells as the pancreas or the pea seedling 

 root (Webster and Johnson, 1955). 



Although little is known about the exact role of nucleic acids, and particularly 

 RNA, in protein formation it has been found that they form an essential part of 

 organized centers in the synthesis of proteins. In differentiated cells the biochemist 

 has associated microsomal activity with protein formation in the cytoplasm. 

 Recent electron microscopic studies have shown these particles to be part of the 

 highly organized endoplasmic reticulum from which the microsomes are presuma- 

 bly liberated on homogenization. In contrast, during early embryonic develop- 

 ment part of the RNA seems to occur unassociated with any cytoplasmic particle. 

 If this "free" RNA is just as important for protein formation as the microsomal 

 RNA, it may present a much better opportunity to investigate its role in the syn- 



Literalure p. 539 



