VIII. PROTEIN SYNTHESIS AND GENE ACTION 395 



is considered to be the ribosome-messenger RNA region with which the 

 transfer RNA interacts). This function could involve intermediates in 

 the ribosome beyond the amino acyl-RNA stage as noted earlier. 



B. INFORMATION TRANSFER IN NUCLEATED ORGANISMS 



In this section again, the main emphasis will be on studies with cell- 

 free systems. Since the nucleus is the site of the hereditary control of 

 protein specificity in nucleated organisms, we shall concern ourselves 

 here with evidence and speculation regarding the method by which the 

 nucleus exerts its control. The controlling role of the nucleus, long a 

 subject of speculation, was first clearly indicated by the work of 

 Hammerling (1953) with the alga Acetabularia. These experiments, ex- 

 periments with enucleated Amoeba (reviewed by Brachet and Chan- 

 trenne, 1956) and human amnion cells (Goldstein et al., 1960), demon- 

 strate (1) that protein synthesis can continue for extended, but not 

 indefinite periods of time in the absence of the nucleus, (2) that some 

 long-term control, particularly of differentiation (as in the maturation 

 of Acetabularia), is under nuclear control. It is impossible, from such 

 experiments, to decide whether any fraction of protein synthesis is not 

 under long-temi nuclear control. Furthermore, it is not clear at the 

 moment whether protein synthesis in enucleated organisms might not 

 be under the control of cytoplasmic DNA. 



1. Synthesis of Ribosomes 



Evidence concerning the site of synthesis of the ribosomes is indeed 

 scanty. Much of the RNA of the nucleus appears to be located in 

 ribonucleoprotein particles. In pea seedling nuclei, for example, 20% of 

 the RNA is found in ribosomes (Rho et al., 1961). Furthermore, the ribo- 

 somes of thymus nuclei are active in protein synthesis (Frenster et al., 

 1960) by pathways which are essentially the same as those followed in 

 cytoplasmic synthesis (reviewed by Allfrey and Mirsky, 1961). It is not 

 clear, however, whether the nuclear ribosomes are in the process of 

 synthesis and are destined to be passed to the cytoplasm, or are engaged 

 in the synthesis of the enzymes and other proteins found within the 

 nucleus. 



Using pea seedlings, Ts'o and Sato (1959) have observed that, after 

 short periods of labeling, the RNA of the nuclear ribosomes was 50-100 

 times more radioactive than the RNA of the cytoplasmic ribosomes. If 

 this is not due to a phenomenally rapid synthesis of ribosomes of nuclear 

 function, it indicates that the ribosomes are synthesized in the nucleus. 

 Schneider (1961) has observed a transfer of radioactivity from the RNA 



