530 D. M. PRESCOTT 



The delay in the appearance of cytoplasmic labelling in each experiment 

 could conceivably be explained in one other way besides the hypothesized 

 transfer of RNA from the nucleus to the cytoplasm. The nucleus might be 

 the exclusive site of some contribution to RNA synthesis which precedes 

 polymerization, i.e. some step in the conversion of nucleoside to nucleoside 

 triphosphate. According to this hypothesis, the delay in cytoplasmic 

 labelling might be considered as a measure of time for the labelled tri- 

 phosphate to be formed in the nucleus and delivered to points of RNA 

 synthesis in the cytoplasm. No evidence has been found in the literature 

 that any such activities are localized in the nucleus. 



These results with Tetrahymena are interpreted as evidence that RNA 

 moves continuously from nucleus to cytoplasm. In view of the very rapid 

 arrival of pH]-cytidine in the nucleus, it seems unlikely that the relatively 

 long lag in the appearance of RNA bound tritium in the cytoplasm could 

 result from cytoplasmic RNA synthesis being delayed until some cytidine- 

 derived precursor of RNA could first difi^use out of the nucleus. 



Initially the nucleus curve for pH]-cytidine incorporation shows a 

 short lag, which probably reflects the time required for pH]-cytidine or a 

 cytidine derivative to be built up in a precursor pool. The slope of the 

 nuclear curve subsequent to the lag does not represent the rate of RNA 

 synthesis in the nucleus but is a composite of rates of several events. 

 During the entire course of the curve the average rate of increase of radio- 

 activity in the nucleus is decreased by cell division, which occurs con- 

 tinuously during the experiment. At each cell division, the activity of the 

 nucleus is divided between the two daughter nuclei, thus lowering the 

 average amount of activity per nucleus. For most of its course the slope of 

 the curve is also decreased by the shift of radioactivity from the nucleus to 

 cytoplasm. The slope of the cytoplasmic curve is also decreased by cell 

 division and possibly by some breakdown of RNA, although the occurrence 

 of the latter seems doubtful. Granting that RNA does move from nucleus 

 to cytoplasm, the lag in the cytoplasmic curve also suggests that there is a 

 delay between the fixation of pH]-cytidine into an acid-insoluble polymer, 

 very probably RNA, and the transfer of the completed RNA-protein 

 molecule into the cytoplasm. 



A number of studies [47] have show^n that incorporation of radio- 

 activity into nuclear RNA during a brief exposure to label is observed to 

 disappear from the nucleus with concomitant appearance of labelled RNA 

 in the cytoplasm when the cells are subsequently transferred to and 

 incubated in a medium containing no label. 



In the second group of experiments, Tetrahymena were exposed to a 

 pulse of pH]-cytidine, and the distribution of labelled RNA followed after 

 removal of exogenous pH] -cytidine. Figure 3 describes the results of the 

 experiment. Ten /xc./ml. of ['^H] -cytidine were added at time zero. The 



