RNA AND CONTROL OF CELLULAR PROCESSES 109 



inside the cytoplasm? Allfrey et al. (1957) believed that other sys- 

 tems may be involved. They were able to show protein synthesis 

 in isolated nuclei and this synthesis was suppressed with desoxyribo- 

 nuclease. The addition of DNA or its degradation products restored 

 the synthesis. Later it was found (Allfrey and Mirsky, 1958) that 

 even some unrelated polxanions could restore protein s\nthesis. 

 Thus, it could not be proven that nuclei have a specific DNA de- 

 pendent protein synthesis. Recentlv, it has been reported that 

 nuclear synthesis requires activated amino acids and soluble RNA, 

 similar to microsomal synthesis (Hopkins, 1959; Rendi, 1960). 



While the nucleus probably does not contribute to the cytoplas- 

 mic protein production, part of the nuclear proteins may be synthe- 

 sized in the cvtoplasm. Goldstein (1958) showed that a protein 

 rich in sulphur, labeled with S''', found in the nucleus, can travel 

 through the cytoplasm and accumulate in another nucleus. It is 

 obvious that nucleus and cvtoplasm should not be taken as two 

 strictly separate entities, but as cell organelles with continuous inter- 

 action and no absolute barrier to prevent exchange of even the larg- 

 est molecules. 



Exchange Reactions. Manv cases have been reported where 

 amino acids were incorporated into proteins in the complete absence 

 of RNA. Proteolytic enzymes are capable of building up protein 

 molecules (Fruton and Simmonds, 1953), and in transpeptidations, 

 individual amino acids or large fractions of protein molecules can be 

 exchanged. It would be imprudent to confuse this kind of amino 

 acid incorporation with the building of a specific protein, unless it 

 could be proved that such reactions could lead to a specific sequence 

 of amino acids in the polypeptide chain. While enzyme chemistry 

 showed that a sequence of enzymatic reactions can lead to the for- 

 mation of specific and often quite complicated molecules, it would 

 be difficult to conceive a set of enzymes able to produce hundreds 



Fig. 4-2. Incorporation of H" leucine and H" glycine into silk glands of 

 Mo/acosomma amencana. Radioactive precursors were injected into young 

 caterpillars, silk glands were prepared and fixed by freeze-substitution at ap- 

 propriate times, sectioned to 4 jx thickness and autoradiographs made. A. H" 

 leucine (3570 mc mmole), fixed after 1 min, exposure time 30 days. B. H '■ leu- 

 cine, fixed after 20 min, exposure time 3 days. C. H"' glycine (44.2 mc mmole), 

 fixed after 1 min, exposure time 30 days. D. H"* glycine, fixed after 20 min, 

 exposure time 3 days. 



