BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 493 



lates the production of the hemolytic toxin called streptolysin S during 

 streptococcal growth: the precise chemical nature of streptolysin S is not 

 yet known, but enzymic digestion experiments show that protein is essential 

 for activity (Bernheimer^"*). 



2. Cellular Mechanisms of Protein Synthesis 

 a. Short Summary of Caspersson's Theory 



In 1940, T. Caspersson^*'^^'"* proposed a general theory of the mechanism 

 of protein synthesis in the intact cell. This theory, as will be seen later, 

 still holds good, although some of its aspects should now undergo modifica- 

 tion. 



Caspersson's theory is based on three fundamental principles :'°^ 



1. All protein synthesis requires the presence of nucleic acids. 



2. Quantitatively the most important nucleic acids in the chromosomes 

 are of the deoxyribose type. 



3. The nucleus itself is a cell organelle especially organized as the main 

 center for the formation of proteins. 



Starting from these premises, Caspersson^*'*^ suggests that the euchro- 

 matin, genetically active and rich in DNA, controls the synthesis of the 

 more complex and specific proteins: these Avould thus be products of the 

 genes. Heterochromatin, especially the nucelolus-associated chromatin, 

 controls the synthesis of histone-like proteins, which are rich in diamino 

 acids. These substances accumulate to form the main bulk of the nucleolus. 

 From the nucleolus, the basic proteins diffuse towards the nuclear mem- 

 brane, cross it, and induce in the perinuclear cytoplasm an intensive 

 production of pentosenucleoproteins, the basic amino acids (arginine and 

 histidine) being precursors of the PNA purines. These cytoplasmic pentose- 

 nucleotides somehow induce the synthesis of cytoplasmic proteins. As 

 indicated in Fig. 1, nucleoli and cytoplasmic PNA are intermediaries be- 

 tween nucleolar-associated chromatin and cytoplasmic proteins. 



Certain aspects of Caspersson's initial theory, which is now more than 

 twelve years old, can hardly be retained at present: this is especially true 

 for the role which the histones are supposed to play. Work by Mirsky and 

 Pollister^"^ and by Mirsky'"* has shown that the absorption spectra of 

 purified histones cannot be distinguished from those of more complex 

 proteins, e.g., albumins. Furthermore, Vincent,'"^ working with nucleoli 

 isolated from starfish oocytes, did not succeed in obtaining any histone-like 



los A. W. Bernheimer, J. Exptl. Med. 90, 373 (1949). 



">« T. Caspersson, Symposia Soc. Exptl. Biol. 1, 127 (1947). 



10' A. E. Mirsky and A. W. Pollister, Proc. Natl. Acad. Sci. U. S. 28, 344 (1942). 



108 A. E. Mirsky, Advances in Enzyniol. 3, 1 (1943). 



'»« W. S. Vincent, Proc. Natl. Acad. Set. U. S..38, 139 (1952). 



