RNA Synthesis in the Nucleus and RNA Transfer to 

 the Cytoplasm in Tetrahymena pyriformis 



D. M. Prescott 



Biology Division, Oak Ridge National Laboratory * 

 Oak Ridge, Tenn., U.S.A. 



Studies of pH]-cytidine labelling in Tetrahymena have been made in 

 regard to two problems, (i) the transfer of RXA between nucleus and 

 cytoplasm, and (2) the nucleus as the major or exclusive site of RNA 

 synthesis. 



In earlier experiments, diverse types of cells have been incubated with 

 pH]-cytidine or other labelled precursors of RNA. The first radioactivity 

 incorporated into RNA has consistently been localized in the nucleus [1-7]. 

 After a measurable time lag, labelled RNA begins to accumulate in the 

 cytoplasm. These experiments have been interpreted as a demonstration 

 of RNA synthesis in the nucleus and the transfer of this molecule to the 

 cytoplasm. With this explanation it has been implied or stated that the 

 nucleus is a principal site of RNA synthesis. The early incorporation of 

 activity into the nucleus certainly does not appear open to any other 

 interpretation than, at the very least, a rapid synthesis of RNA in that 

 location. 



The three types of experiments were : 



1. A time-study of RNA synthesis in the nucleus and cytoplasm with 

 [•^H]-cvtidine continuously present in the medium. 



2. A study of the pattern of labelling in the nucleus and cytoplasm after 

 a short exposure to [^H]-cytidine. 



3. Investigation of the capacity of nucleated and enucleated cells to 

 incorporate [•'^H]-cytidine into RNA. 



All three experiments lead to one general conclusion; all RNA is syn- 

 thesized in the nucleus, and cytoplasmic RNA is of nuclear origin. 



Figure i shows the pattern of [^H]-cytidine accumulation into nuclear 

 and cytoplasmic RNA of Tetrahymena. In the first group of experiments, 

 10 fxc.'ml. of [^H]-cytidine were added to an early log phase culture. At 

 intervals of a few minutes, groups of cells were withdrawn, dried on slides, 



* Operated by Union Carbide Corporation for the U.S. Atomic Energy 

 Commission. 



