EXPERIMENTAL RESULTS 103 



enucleate Amoeba halves are obviously compatible with the idea 

 that cytoplasmic RNA originates from nuclear RNA. They do not, 

 however, prove that this is so, for RNA might disappear from 

 enucleated cytoplasm for a variety of reasons other than that of a 

 nuclear origin. More direct experiments have been designed by 

 Goldstein and Plaut (1955) to prove the nuclear origin of cyto- 

 plasmic RNA. These workers labeled amoebae strongly with ^ap 

 by feeding them with Tetrahymena cultivated in the presence of 

 radiophosphate. The nucleus of the tagged amoebae was then 

 removed and grafted into normal unlabeled amoebae or into enu- 

 cleate halves. Autoradiographs showed clearly that the cytoplasm 

 of the grafted amoebae becomes radioactive after 12 or more hours. 

 Utilization of the ribonuclease test further showed that, under the 

 conditions adopted for the autoradiography (fixation in 45 % acetic 

 acid), all the autoradiographically detectable ^^P in both the nucleus 

 and the cytoplasm is in the form of RNA. When the tagged nucleus 

 is grafted into a whole amoeba so as to produce a binucleate cell, 

 it is found that the originally unlabeled nucleus does not acquire 

 any significant amount of radioactivity. This last experiment shows 

 that the cytoplasm does not supply RNA to the nucleus, which, 

 therefore, must synthesize its own RNA. Once synthesized in the 

 nucleus, RNA can be transferred to the cytoplasm and the transfer 

 proceeds in that direction only. 



These experiments of Goldstein and Plaut (1955) are of far- 

 reaching importance for our understanding of the interactions 

 between nuclear and cytoplasmic RNA. They certainly deserve a 

 short critical discussion. There is no doubt that Goldstein and 

 Plaut (1955) have good evidence for the view that RNA is syn- 

 thesized in the nucleus and transmitted therefrom to the cytoplasm. 

 However, as pointed out by the authors themselves, they have not 

 proved that the labeled material rnigrating to the cytoplasm is the 

 RNA as it actually existed in the nucleus; it might be a precursor 

 of a more or less complex nature. Goldstein and Plaut's (1955) 

 demonstration, that the labeled RNA (or its precursor) which has 

 been passed from the nucleus into the cytoplasm cannot be used for 

 further nuclear RNA synthesis, does not seem entirely convincing. 



References p. 1331135 



