RNA AND CONTROL OF CELLULAR PROCESSES 127 



nature, aiid is eventuallv destroyed bv cellular enzymes. In the 

 latter case, the relatiye amounts of RNA present would remain en- 

 tirely under direct genie control. 



The RNA which mediates protein synthesis in the cytoplasm does 

 not function as long as it is still inside the nucleus, as lias been 

 shown in silk production ( see aboye ) . The lack of cytoplasmic RNA 

 actiyity before it leayes the nucleus cannot be due to the lack of 

 necessary precursors, since nuclear RNA is able to synthesize pro- 

 teins. RNA may haye to be combined first with a protein to form 

 a nucleoprotein particle, in order to become actiye. This probably 

 happens inside the nucleolus which seems to store inactiye RNA for 

 some time. Thus, cellular actiyity could be controlled by the timing 

 of the nucleolar release of RNA particles, but this release could 

 hardly be specific for only one type of RNA. In secretory glands, 

 such a release of nucleolar material has been actually obseryed dur- 

 ing actiyation by pilocarpin (Altmann, 1952). 



It would appear that RNA becomes functional as soon as it leayes 

 the nucleus, as was postulated by Caspersson ( 1941 ) , who obseryed 

 the accumulation of RNA and protein synthesis in the nuclear pe- 

 riphery. Howeyer, there are cells containing high amounts of RNA 

 and still not producing a large amount of protein. This is particu- 

 larly true for "resting" reproductiye cells. Fungus conidia, such as 

 those of Neurospora, are rich in RNA (Minagawa et al, 1959), yet, 

 for a period of time they do not grow, their metabolism is at a low 

 level, and certainly they do not produce any new proteins. In the 

 ovocytes of many animals, RNA accumulates before the start of the 

 most intensive protein production. In Acetabiilaria, morphogenesis 

 can proceed in the absence of the nucleus and therefore, in the 

 absence of RNA synthesis (barring the possibility of extranuclear 

 RNA synthesis, see above), provided that nuclear morphogenetic 

 products were previously accumulated in sufficient quantities ( Haem- 

 merling, 1953). It would be reasonable to assume that this mor- 

 phogenesis requires the production of new kinds of proteins. RNA 

 for the production of these proteins must have been present in the 

 cell all along but must have become active only to form specific 

 structure. 



It has been suggested (Tissieres and Watson, 1958) that the size 

 of RNA particles may reflect their functional state. McQuillen et al. 

 (1959) found, indeed, b\ isolating ribosome fractions of bacteria 

 after feeding radioactive amino acids, that the highest incorporation 



