RNA AND CONTROL OF CELLULAR PROCESSES 91 



of nuclei 1)\' a mitochondrial layer. Altlioiigli the rihosome layer 

 contained most of the cellular RNA, all radioactivity newly incor- 

 porated into RNA was found in the nuclear layer for the first few 

 minutes. After four minutes, more and more radioactivity appeared 

 in the rihosome layer until, after one hour, the ribosome fraction was 

 labeled much more than the nuclear fraction. Later, similar experi- 

 ments were made with H'^ cytidine and C^^ adenine, with essentially 

 the same results. Kinetic studies of uridine and adenine uptake into 

 the cell excluded another possible explanation, that of a delayed 

 RNA SMithesis in ribosomes. Administration of radioactive precur- 

 sor for a brief time, followed bv an excess of non-radioactive pre- 

 cursor, proved that it must have been actually RNA which migrated 

 from the nuclear into the ribosome fraction. The amount of precur- 

 sor incorporated in these experiments corresponded roughly to the 

 amount of RNA formed in the same period, thus excluding the pos- 

 sibility of unspecific exchange reactions. 



The nuclear origin of RNA was confirmed also in autoradiographic 

 experiments with the salivary glands of Drosophila (Taylor et al., 

 1955; Zalokar, 1960a), with mammalian tissue cultures (Goldstein 

 and Micou, 1959a; Feinendegen et al, 1960) and with plant cells 

 (Woods, 1959; Woods and Taylor, 1959). Then came the disturb- 

 ing report of Harris ( 1959 ) who obtained contrary results in tissue 

 cultures. He fed H' cvtidine or H' adenosine to rabbit macrophages 

 and rat heart fibroblasts, freshly subcultured from an animal. The 

 first survived in culture as undividing cells. They incorporated large 

 quantities of adenosine in the nuclei and nucleoli, less in the cyto- 

 plasm. After transfer to a non-radioactive medium, the labeling of 

 nuclei and nucleoli disappeared rapidly, and there was no indication 

 that this RNA passed into the cytoplasm. Connective tissue cells, 

 multiplying in vitro, incorporated adenosine rapidly also, but after 

 the transfer to the non-radioactive medium, nuclear labeling re- 

 mained unchanged for about two hours and cytoplasmic labeling 

 continued to increase. After that time, the nuclear labeling de- 

 creased rapidly, while the cytoplasmic labeling remained at a con- 

 stant level. Again, these results did not indicate that appreciable 

 amounts of the nuclear RNA could pass intact into the cytoplasm. 

 The author concluded that there must be a rapid turnover of RNA 

 in the nuclei and nucleoli, but that this RNA does not enter the cyto- 

 plasm intact. Only a small proportion of the nuclear RNA could 

 liave passed into the cytoplasm without being detected. 



