534 D. M. PRESCOTT 



ceases completely, although the enucleated pieces remain motile long 

 beyond this time. 



The cutting experiments show that the cytoplasm is completely 

 incapable of incorporating pH]-cytidine into RNA when the nucleus is 

 absent from the system. As in other enucleated cell types cytoplasmic 

 incorporation of amino acids continues. These enucleation experiments 

 with Tetrahymeno, combined with the time-labelling study (Fig. i) and 

 the pulse experiment (Fig. 3) substantiate the thesis that all RNA is 

 synthesized in the nucleus and that nuclear RNA is the source of cyto- 

 plasmic RNA. The fate of "transfer RNA" (soluble RNA) in all of these 

 experiments is not entirely certain ; it is possible that some or all of this 

 fraction might have been lost during the extraction procedures prior to 

 autoradiography. For this reason, cytoplasmic labelling of transfer RNA 

 with [-'HJ-cytidine might, therefore, have escaped detection. 



References 



1. Prescott, D. M., Exp. Cell Res. 12, 196 (1957). 



2. Woods, P. S., and Taylor, J. H., Lab. Invest. 8, 309 (1959). 



3. Goldstein, L., and Micou, ].,y. biophys. biocheni. Cytol. 6, 301 (1959). 



4. Woods, P. S., Brookhaven Symp. Biol., Number 12, 153 (1959). 



5. Zalokar, M., Exp. Cell Res. 19, 559 (i960). 



6. Perry, R. P., Exp. Cell Res. 20, 216 (i960). 



7. Amano, M., and Leblond, C. P., Exp. Cell Res. 12, 196 (i960). 



8. Goldstein, L., and Plaut, W., Proc. nat. Acad. Set., Wash. 41, 874 (1955). 



Discussion 



Davls: Your curve showed considerable increase in total counts in the cell if 

 you add together the nucleus and cytoplasm after division has started. How can 

 you account for the source of all that further radioactivity ? 



Prescott : We can account for this because the cells contain a pool of cytidine 

 and its derivatives that cannot be washed out of living cells. Dr. Bruce Jacobson and 

 I have started some studies on this pool in Tetrahymena and we already know that 

 it is in fact very large. We haven't really identified it yet but suspect that it is com- 

 posed of CMP, CDP, and CTP. 



Davis : I missed the point of technique ; didn 't your method of autoradiography 

 involve getting rid of that pool first ? 



Prescott: Yes, I am sorry that I didn't make that clear. The cells were pre- 

 pared for autoradiography in such a way as to remove all acid-soluble material and 

 to give a picture of incorporation only. 



Chargaff : Does Tetrahymena contain cytidine deaminase ? 



Prescott: I don't know. 



Chargaff : Because if you rely on grain counts you do not know that it is still 

 cytosine derivatives that you are still following. 



