ROLE OF NUCLEIC ACIDS 87 



it might possibly be due, e.g. to chain end metaboHsm (see p. 66). Direct 

 determination of the RNA content of batches of nucleate and enucleate 

 parts of Acetahidaria indicated a net RNA synthesis of short duration in 

 enucleate fragments (Brachet et al., 1955). Other groups of workers 

 observed an immediate cessation of RNA synthesis after enucleation 

 (Richter, 1957; Noara et al, 1959), or even a loss of RNA. It is probable 

 therefore that the RNA found in the cytoplasm comes largely from the 

 nucleus. On the other hand, cytoplasmic RNA in Acetahularia is not inert, 

 it seems to undergo a turnover process (degradation and resynthesis), which 

 is independent of the nucleus. When this cytoplasmic RNA metabolism is 

 interfered with by ultraviolet irradiation, net loss is observed. This is, how- 

 ever, followed by recovery and a net synthesis of RNA is then observed in 

 enucleate fragments (Richter, 1959). Conflicting results obtained by differ- 

 ent groups are probably due to differences in physiological conditions of 

 the algae under study in the individual cases. Different adjustment of the 

 cytoplasmic steady state of RNA may result in net synthesis (Brachet et ah, 

 1955), absence of variation (Richter, 1957) or net decrease of RNA content 

 of the cytoplasm (Naora et al., 1959). Chloroplastic RNA forms in enucleate 

 cytoplasm, apparently at the expense of other cytoplasmic RNAs (Naora 

 et al., 1960). The cytoplasmic RNA turnover observed by Richter (1959) 

 also explains that incorporation of various precursors into cytoplasmic 

 RNA is largely independent of the nucleus. All the data on net synthesis of 

 RNA in Acetahularia nevertheless make it clear that this process is closely 

 dependent on the presence of the nucleus, and much more so than protein 

 synthesis which continues undisturbed after enucleation. Recent results by 

 Richter (1959) are especially convincing: if a nucleate fragment oi Acetahul- 

 aria is grafted on an enucleate part in which net RNA synthesis had 

 stopped, the amount of RNA again starts to increase in the old cytoplasmic 

 fragment. 



This is all very suggestive of a nuclear production of RNA which is 

 secreted into the cytoplasm. The important point would be to know whether 

 this special RNA fraction is bringing with it information for protein syn- 

 thesis. Although this question cannot be answered yet, interesting results 

 relevant to this question were reported by Stich and Plant (1958). Ribo- 

 nuclease inhibits protein synthesis both in nucleate and enucleate frag- 

 ments of Acetahularia. When the fragments are transferred to normal 

 medium after a few days of ribonuclease action, protein synthesis is 

 restored in nucleate but not in enucleate moieties. This shows that a sub- 

 stance which is inactivated by ribonuclease and which is involved in protein 

 synthesis requires the presence of the nucleus for its formation. It is exactly 

 what would be expected if the 'central dogma' was true. 



In Amoeha, the importance of the nucleus for the synthesis of cyto- 

 plasmic RNA is even more striking than in the case of Acetahularia. Thus 



G 



