14 J. Bracket 



Acetabular ia stems, has the chemical composition of the nuclear 

 RNA or that of the cytoplasmic RNA, provided the two differ 

 in this respect; for the essential fact remains that the non- 

 nucleated Acetabularia mediterranea is capable of forming a 

 specific mediterranea regenerate in the absence of a nuclear 

 production of either RNA or DNA. It is unlikely that the 

 morphogenetic substance produced by the Acetabularia 

 nucleus is DNA, since we have been unable to detect the 

 presence of DNA in the non-nucleated stems with a sensitive 

 isotope dilution method. 



The observed differences, with respect to protein and RNA 

 synthesis in non-nucleated cytoplasm, between Amoeba and 

 Acetabularia are not altogether unexpected if we consider 

 that the non-nucleated stem of an Acetabularia retains, as we 

 have seen for ourselves, a perfectly normal photosynthetic 

 activity. Thus the energy requirements for nucleoprotein 

 synthesis are still met with in a non-nucleated piece of 

 Acetabularia, but not in a non-nucleated Amoeba half. 



We must finally point out the very clear correlation which is 

 found in both Amoeba and Acetabularia between the fate of 

 RNA and that of the proteins. In the former, removal of the 

 nucleus is followed by a rapid loss of RNA and a marked 

 decrease in protein metabolism. In Acetabularia, on the con- 

 trary, both processes are accelerated in a parallel manner. 

 Those are by no means special cases, since what we have just 

 said for Acetabularia applies also to reticulocytes. These are 

 immature red cells which have lost their nucleus, but still 

 retain a nearly normal amount of RNA in the cytoplasm. 

 They still have the power of incorporating tagged amino acids 

 into their proteins, an activity which is completely lacking 

 in mature erythrocytes. The latter have almost completely 

 lost their RNA (Borsook et al., 1952; Koritz and Chantrenne, 

 1954; Holloway and Ripley, 1952). The same reticulocytes, 

 in spite of the loss of their nucleus, can also incorporate 

 radioactive glycine in their RNA, as shown recently by Kruh 

 and Borsook (1955); these anucleate cells can even synthesize 

 haemoglobin (Nizet and Lambert, 1953) and various enzymes 



