EXPERIMENTAL RESULTS 125 



removal of the nucleus, there is no change in the RNA content 

 while there is a net synthesis of proteins; this indicates that, as in 

 amoebae, the nucleus exerts a more direct control on RNA than on 

 proteins. This can be explained if we assume that the nucleus is 

 directly involved in the production of cytoplasmic RNA: protein 

 synthesis would stop when the stock of RNA of nuclear origin 

 is exhausted. We shall now see whether these ideas and conclusions 

 are valid for other cells, e.g. reticulocytes and eggs. 



d. RNA and protein synthesis in the absence of the nucleus in retic- 

 ulocytes and eggs 



The reticulocytes (immature red blood cells which have lost their 

 nucleus and retained their RNA in the form of a basophilic net- 

 work) have been extensively studied in Borsook's laboratory. It is 

 now well-established (Borsook et ah, 1952; Koritz and Chantrenne, 

 1954; Holloway and Ripley, 1952) that the reticulocytes are capable 

 of incorporating amino acids into their proteins, including hemo- 

 globin, despite the loss of their nucleus. In contrast, adult red blood 

 cells have practically lost the ability to incorporate amino acids 

 into proteins, and they contain traces only of RNA. According to 

 Holloway and Ripley (1952), the development of reticulocytosis is 

 accompanied by a substantial increase in the RNA content, which 

 is closely paralleled by the amount of radioactive leucine incor- 

 porated into the proteins. The authors point out that their results 

 are compatible with the view that RNA is closely associated with 

 amino acid incorporation into proteins. It should be added that 

 they are not compatible with the opinion that the cell nucleus is the 

 most important center of protein synthesis. 



Slightly different results have, however, been reported by Koritz 

 and Chantrenne (1954), who found that the maximal rate of incor- 

 poration of labeled glycine precedes the RNA maximum by 2 or 

 3 days. The RNA peak coincides with the maximal content of the 

 red blood cells in hemoglobin, dipeptidase and carbonic anhydrase. 

 Since proteases, peptidases and phosphatases increase during retic- 

 ulocytosis (Ellis et al., 1956), it is quite possible that the enucleate 

 reticulocytes are capable of specific enzyme synthesis. In fact, some 



References p. 133/135 



