EXPERIMENTAL RESULTS 115 



the absence of the nucleus. Finally, these isotope experiments have 

 confirmed that the synthesis of RNA is practically confined to the 

 chloroplasts. 



Thus, the present situation is as follows. RNA anabolism is pre- 

 dominant in the nucleolus, but the chloroplasts, which have often 

 been believed to be cytoplasmic self-duplicating units, can synthe- 

 size RNA in the absence of the nucleus. The other cell fractions 

 (microsomes, supernatant liquid) are, as in amoebae, dependent on 

 the cell nucleus for the synthesis and even for the maintenance, of 

 their RNA store. 



It is thus probable that two distinct phenomena co-exist in Acetab- 

 ularia as in Amoeba: independent cytoplasmic RNA synthesis 

 (especially in the chloroplasts) occurs along with transfer to the 

 cytoplasm of nuclear RNA. 



In order to test the importance of RNA from nuclear origin for 

 growth, morphogenesis and protein synthesis, ingenious experi- 

 ments have been performed by Stich and Plaut (1958). They treated 

 nucleate and enucleate fragments with ribonuclease for a few days 

 and then returned them to normal sea water. After a while, the 

 nucleate fragments started to regenerate and synthesize proteins; 

 but nothing of the sort happened in the enucleate fragments. Stich 

 and Plaut's (1958) conclusion is therefore that the integrity of RNA 

 of nuclear origin is required to ensure regeneration and protein 

 synthesis. Attempts to repeat these interesting experiments (Brachet, 

 1959 and unpublished) failed to give such definite results, the main 

 difficulty being to avoid the infection of the ribonuclease-containing 

 solution. It could, however, be confirmed that ribonuclease really 

 penetrates into the algae and that it exerts a stronger inhibitory 

 action on the regeneration of the enucleate fragments than on that of 

 their nucleate counterparts. Furthermore, it was found by the author 

 (unpublished) that 5,6 dichloro-/?-D-ribofuranosyl-benzimidazole, 

 a substance which strongly inhibits RNA synthesis in mammalian 

 cells, has also a stronger inhibitory effect on the regeneration of the 

 enucleate halves. Finally, it was observed that normal purine bases 

 (especially adenine), which are so quickly incorporated into nucleolar 

 RNA, strongly inhibit the growth of normal algae (Brachet, 1959). 



References p. 133/135 



