EXPERIMENTAL RESULTS 105 



halves ; the whole of cytoplasmic RNA would thus, in amoebae, 

 originate from the nucleus. 



The reason for this discrepancy is not yet clear, but it might 

 simply be due to the fact that uracil is perhaps not as easily incor- 

 porated into RNA as adenine. In fact, the incorporation of adenine 

 itself into the RNA of amoebae is disappointingly low and, for this 

 reason, incorporation of ^'*COo into RNA was studied. This pre- 

 cursor is very well incorporated into amoebae but its specificity is 

 of course very low. The results obtained in this investigation (Tencer 

 and Brachet, 1958; Brachet, 1959), in which both autoradiography 

 and counting techniques were used, confirm the data previously 

 obtained with adenine. Removal of the nucleus strongly reduces 

 (by a factor of 2 to 3) the incorporation of ^^COg into RNA; but 

 easily measurable incorporation of ^'*COo into RNA can be detected 

 even in fragments which have been devoid of the nucleus for 8 days. 

 It is thus likely that some synthesis of RNA is still possible in the 

 prolonged absence of the nucleus. Furthermore, the fact that such 

 a simple precursor as carbon dioxide can be utilized by enucleate 

 fragments clearly shows that very complex metabolic processes 

 can go on in amoebae, even in the absence of the nucleus, for a con- 

 siderable time. 



The conclusion is that the nucleus of amoebae is of considerable 

 importance for RNA metabolism ; in its absence, the RNA content 

 of the cytoplasm drops markedly. Since Goldstein and Plaut's 

 (1955) work, there is good evidence that the nucleus actively syn- 

 thesizes RNA and that nuclear RNA is transferred to the cytoplasm. 

 But the cytoplasm is not entirely inactive, and a limited synthesis or 

 turnover of RNA can continue in enucleate cytoplasm. Such a pic- 

 ture is, of course, in very good agreement with Mazia's (1952) re- 

 placement hypothesis. 



If removal of the nucleus produces a marked weakening of RNA 

 metabolism in Amoeba, one would expect a parallel inhibition of 

 protein anabolism, if both are really closely linked together. In 

 accordance with this expectation, the total protein content was 

 readily found to drop more quickly in enucleate halves than in 

 nucleate fragments (Brachet, 1955). This finding raised a new 



References p. 133/135 



