88 THE BIOSYNTHESIS OF PROTEINS 



the RNA content of enucleate fragments of Amoeba proteus decreases 

 quickly and strongly during starvation whereas it is maintained in nucleate 

 halves (Brachet, 1955). If living Amoebae are transferred into a medium 

 containing ribonuclease, the enzyme enters the cells and destroys a large 

 part of the RNA both in the cytoplasm and in the nucleus. When the 

 Amoebae are later returned to normal medium or better to a medium con- 

 taining yeast RNA, nucleic acid is reformed in many Amoebae; under such 

 conditions, RNA appears first in the nucleoli and later in the cytoplasm, 

 and enucleate fragments are unable to restore their nucleic acids (Brachet, 

 1955 b, c). 



More direct evidence for the passage of nuclear RNA into the cyto- 

 plasm in Amoeba proteus was obtained by Goldstein and Plant (1955). 

 Amoebae were fed with micro-organism which had been grown in a medium 

 containing 22PO4. The nuclei of labelled Amoebae were grafted into non- 

 labelled Amoebae. Autoradiography studies and ribonuclease test indicated 

 that radioactive phosphorus contained in the nuclear RNA at the beginning 

 of the experiment was lost by the nucleus and later found in cytoplasmic 

 RNA. Although in these experiments it is actually the fate of phosphorus 

 atoms which was studied, not that of RNA molecules, these data strongly 

 suggested that part of the cytoplasmic RNA originates in the nucleus. 



Incorporation experiments using radioactive adenine and uracil in 

 enucleate cytoplasm of Amoeba invariably showed that enucleation drastic- 

 ally reduces the incorporation into cytoplasmic RNA (Plaut and Rustad, 

 1956; Prescott, 1957). Conflicting results have been reported as to whether 

 cytoplasm can make some RNA in the absence of the nucleus or whether 

 all cytoplasmic RNA comes from the nucleus. The latter opinion was 

 advocated by Prescott (1957, 1959) who believes that the low residual 

 incorporation observed in enucleate Amoeba proteus is due to the activity 

 of micro-organisms recently phagocyted by the Amoeba. Indeed with an 

 Acanthamoeba, which can be grown in sterile medium, no detectable incor- 

 poration of adenine, orotic acid or uracil takes place in enucleate fragments 

 (Prescott, 1960). An uncertainty which remains is whether the precursors 

 enter the enucleate parts at all, for absorption of various substances can be 

 afl^ected by enucleation (see Mazia and Prescott, 1955). 



Recent kinetic studies on RNA synthesis in populations of animal cells 

 also provide evidence that some RNA of nuclear origin can be secreted 

 into the cytoplasm. Thus cells of the human amnion grown in tissue culture 

 were exposed for a short time to cytidine-^H, and then replaced in a 

 medium containing non-labelled cytidine. Autoradiography showed a 

 progressive movement of the label from nucleus to cytoplasm. All the 

 label that was in the RNA of the nuclei at the time of transfer into 'cold' 

 medium was found in cytoplasmic RNA 24 hr later (Goldstein and Micou, 

 1959). 



