RNA AND CONTROL OF CELLULAR PROCESSES 97 



plasm so that it never reaches appreciable concentrations in the 

 nucleus. 



Experiments with Neurospora (Zalokar, 1960h) showed that all 

 the RNA synthesized in the nucleus is not emptied into the cyto- 

 plasm. When H^ uridine was added for one minute and tlien re- 

 placed with non-radioactive uridine, radioactivity due to RNA 

 remained in the nuclear fraction as much as one hour later. In 

 Drosophila, fed P"'- for one hour and non-radioactive pliosphorus 

 subsequently, radioactivity did not disappear completely from the 

 nucleus (Ta\'lor et ah, 1955). On the other hand, Goldstein and 

 Micou ( 1959b ) could demonstrate in tissue culture cells the essen- 

 tially complete disappearance of radioactivity from the nuclear frac- 

 tion when a non-radioactive precursor followed a short treatment 

 with the tracer. These differences can be explained by the fact that 

 the nuclei in Neurospora are multiplying, and in Drosophila increas- 

 ing in size, therefore continuously adding to their own substance, 

 part of which is nuclear RNA. In tissue culture, the nuclei may have 

 been active mainly in the production of cytoplasmic RNA and there- 

 fore would not have increased their own substance. 



Although most of the nuclear RNA seems to be inside the nucle- 

 olus, careful consideration of different measurements shows that the 

 amount of RNA in the rest of the nucleus is as large or even larger 

 ( Vincent, 1955 ) . The nucleolar RNA obtained from isolated nucle- 

 oli was shown to be different from the cytoplasmic RNA (Vincent, 

 1952), and this may account for the main part of the nucleus-specific 

 RNA. About one third of the total synthesis of RNA in the nucleus 

 of the salivary glands of Drosophila appears to occur in the nucleoli, 

 and it may well be that it is mainlv the specific nucleolar RNA which 

 is made in the nucleolus. This finding does not contradict the hy- 

 pothesis of the genie origin of RNA: the nucleoli are themselves of 

 chromosomal origin, as was clearly demonstrated by McClintock 

 ( 1934) in her work on the nucleolar organizer. The nucleolar RNA 

 must have some special function in the nucleus and is, therefore, 

 produced in relatively large quantities, the production still depend- 

 ing on the continuous activity of one or a few gene loci. 



Another chemically well-defined RNA is the so-called soluble RNA, 

 of importance in the transfer of activated amino acids for protein 

 synthesis (Hoagland et ah, 1957). While gene-produced RNA is 

 expected to carry information on the sequence of many amino acids 

 in the protein molecule, the soluble RNA molecule is specific only 



