104 ROLE OF THE CELL NUCLEUS 



Proof that the RN A transfer can only proceed in one direction, i.e. 

 from the nucleus toward the cytoplasm, would be more complete 

 if it had been shown that an unlabeled nucleus grafted in a strongly 

 labeled enucleate cytoplasm never becomes radioactive. In Gold- 

 stein and Plant's (1955) experiments on binucleate amoebae, the 

 radioactivity of the cytoplasm seems to be rather weak and no un- 

 equivocal answer can be obtained from their observations. 



An important additional remark was made by Goldstein and 

 Plaut (1955) who noted that the possibility of the complete syn- 

 thesis of some RNA in the cytoplasm is not ruled out by their data. 

 In other words, besides a transfer of nuclear RNA to the cytoplasm, 

 independent synthesis may occur in the latter. This seems to be 

 probable in view of autoradiographic experiments performed in 

 this laboratory by Skreb-Guilcher (1955). Studying the incorpora- 

 tion of labeled adenine in nucleate and enucleate Amoeba halves, 

 she obtained a measurable incorporation into the RNA of enu- 

 cleated fragments separated for 1 and 3 days. The activity of these 

 fragments was, however, about four times less than that of the 

 nucleate halves. The incorporation in enucleate cytoplasm became 

 negligible when, 8 days after the section, its RNA content had 

 dropped markedly. Similar results have been reported by Plaut and 

 Rustad (1956) who studied the uptake of adenine into Amoeba 

 fragments and found that it is an effective RNA precursor 

 in this organism. Their experiments show that the presence of 

 the nucleus is not essential for the uptake of adenine; but the 

 nucleus is important for this uptake during the early incubation 

 period, since the ratio between the nucleate and enucleate halves 

 is 2.1 : 1. 



More recently, Plaut and Rustad (1957) have reported results 

 which agree perfectly well with those of Skreb-Guilcher (1955); 

 using radio-active adenine as a label, they found that there is a 

 mechanism of cytoplasmic incorporation into RNA which can 

 operate in the absence of the nucleus. 



Nevertheless, recent autoradiography experiments of Prescott 

 (1957) lead to different results and conclusions. According to him, 

 there is no incorporation whatsoever of labeled uracil in enucleate 



