12 J. Bracket 



Parallel studies done on the unicellular alga Acetabularia 

 mediterranea, an organism which can regenerate to a sizeable 

 extent when deprived of its nucleus (Hammerling, 1934), have 

 yielded quite different results. As we have shown recently 

 (Brachet, Chantrenne and Vanderhaeghe, 1955), Acetabularia 

 mediterranea behaves much like the amoeba as far as respira- 

 tion is concerned. In both, removal of the nucleus has no 

 measurable effect on cellular oxidations, showing that the 

 latter are not under direct nuclear control. Indeed, the reverse 

 appears to be true to some extent in Acetabularia. The 

 morphology and chemical composition of the nucleus are 

 influenced by the cytoplasm. If energy production in the 

 cytoplasm is diminished or blocked by dinitrophenol or an- 

 aerobiosis, the nucleolus soon changes its shape, losing in the 

 process some of its high content of RNA. Such a nucleolar 

 reaction has been observed before by Stich (1951) as a result 

 of merely placing the algae in the dark. 



We have, however, noted a difference between Amoeba and 

 Acetabularia for phosphorus metabolism. While it is true 

 that ^^P incorporation decreases in non-nucleated Aceta- 

 bularia stems, this effect does not become noticeable until 

 after a long time (Brachet, Chantrenne and Vanderhaeghe, 

 1955), usually several weeks. Fragments separated for only 

 a few days show no significant differences in this respect 

 (Hammerling and Stich, 1954). 



It is, however, with respect to RNA and protein metabolism 

 that Acetabularia shows most difference from what has been 

 reported for Amoeba. We have recently been able to show 

 (Brachet, Chantrenne and Vanderhaeghe, 1955) that the non- 

 nucleated stem of Acetabularia retains for several weeks the 

 ability to incorporate radioactive ^^COg into proteins (in the 

 light) and orotic acid into RNA. These anabolic processes 

 continue at a normal rate for fifteen days in non-nucleated 

 cytoplasm. Together with regenerative potency, they then 

 begin to diminish. Even after three months without a nucleus, 

 fragments will still be capable of a noticeable uptake of radio- 

 active precursors into RNA and proteins. 



