116 ROLE OF THE CELL NUCLEUS 



Taken together, all these facts lend support to the hypothesis of 

 Stich and Plant (1958). As in other cells, RNA would be mainly 

 built in the nucleus and part of the cytoplasmic RNA would be of 

 nuclear origin. This part would be the more important in regenera- 

 tion. On the other hand, it is certain that independent cytoplasmic 

 RNA synthesis and turnover are quantitatively much more im- 

 portant in Acetabularia than in Amoeba and the main reason for 

 this difference is that only the former organism contains chloro- 

 plasts. 



Let us now consider protein metabolism in Acetabularia. In the 

 course of regeneration in Acetabularia, the growth of the enucleate 

 fragment is paralleled by increases in wet weight and in protein 

 nitrogen (Vanderhaeghe, 1954; Brachet et al, 1955). If regeneration 

 occurs under suboptimal conditions in which the stalks increase in 

 length but form no or few caps, the rate of protein synthesis is the 

 same in the nucleate and in the enucleate pieces for 1 to 2 weeks. 

 Protein synthesis then stops altogether and alterations of the chlo- 

 roplasts begin; they retain their chlorophyll, but their proteins are 

 partially degraded. In contrast, the small granules (microsomes) 

 remain quantitatively unaffected during this second period (Van- 

 derhaeghe, 1954). 



If, as in the experiments of Brachet et al. (1955), the algae are 

 operated on just before the formation of the caps and if the frag- 

 ments are placed under optimal culture conditions, the enucleated 

 pieces form a high proportion of caps. As a result, net protein syn- 

 thesis is definitely /a5/^r in the enucleate halves than in the nucleate 

 rhizoids (Fig. 37, p. 121). These experiments, which have been con- 

 firmed by Hammerling et al (1959), clearly show that the presence 

 of the nucleus is not necessary for protein synthesis, although it is 

 required for prolonged protein synthesis ; this completely stops after 

 2 to 3 weeks, that is, when the growth of the cap has ceased. There- 

 fore, in Acetabularia, the rate of protein synthesis is initially in- 

 creased by the removal of the nucleus. 



Of great interest regarding the role of the nucleus in protein syn- 

 thesis are recent observations by Werz (1957) who has studied 

 the effects of adding nuclei from either the mediterranea (med) or 



