124 ROLE OF THE CELL NUCLEUS 



much more in enucleate than in nucleate fragments. This increased 

 synthesis of soluble nitrogenous compounds might correspond to 

 an increase in the pool of the precursors for RNA and protein syn- 

 thesis, when the competitive influence of the nucleus has been ex- 

 perimentally removed. A study of the chemical nature of this pool, 

 which probably consists largely of amino acids, peptides and nu- 

 cleic acids derivatives, might be rewarding; the analysis should be 

 relatively easy by chromatographic methods. 



Autoradiographic observations on the incorporation of labeled 

 amino acids into nuclear proteins of Acetabularia (Vanderhaeghe, 

 1957) confirm the idea that considerable uptake occurs in the nu- 

 cleus itself. The proteins of the nucleolus certainly become labeled 

 to a measurable extent before those of the cytoplasm. But, as in 

 amoebae, the difference in the incorporation activity between the 

 nucleus and the cytoplasm is much less striking for the incorporation 

 of an amino acid into proteins than for that of adenine into RNA. 



Before we leave Acetabularia for other topics, a last and im- 

 portant remark should be made. We have mentioned many times 

 in this chapter the control exerted by the nucleus on cytoplasmic 

 activities; but it would be a mistake to forget that the nucleus also 

 lies under cytoplasmic control. This fact has been shown very 

 clearly by Stich (1951). If the algae are placed in the dark for some 

 weeks, the volume of the nucleus and the nucleolus decreases; the 

 latter becomes spherical instead of ribbon-shaped and loses much 

 of its RNA. These changes are perfectly reversible when the algae 

 are returned to light. Similar results have been obtained by Brachet 

 (1952), when he treated algae with well-known poisons of oxidative 

 phosphorylation such as dinitrophenol and usnic acid (Figs. 38 

 and 39, pp. 122 and 123). There is thus no doubt that energy pro- 

 duction in the cytoplasm (by the chloroplasts and the mitochondria) 

 is an essential factor for the regulation of the morphology and the 

 chemical composition of the nucleus and, in particular, the nu- 

 cleolus. 



To summarize the above material, the experiments made on 

 Acetabularia show that enucleate cytoplasm can retain its RNA 

 content and synthesize proteins including specific enzymes. After 



