128 ROLE OF THE CELL NUCLEUS 



than into cytoplasmic RNA. On the other hand, nuclear proteins, 

 even the so-called "residual proteins" which are the most active 

 proteins of the nuclei, incorporate labeled amino acids at a rate 

 comparable to that found for mixed cytoplasmic proteins (Daly 

 et aL, 1952; Allfrey et aL, 1955, Smellie et al., 1953; etc.). Thus the 

 residual proteins are less active than some of the cytoplasmic pro- 

 teins, e.g. those of the microsomes. It can be concluded from this 

 brief summary (see Brachet, 1957, for more details) that these in 

 vivo experiments agree very well with all we have seen previously; 

 the nucleus is more active and thus more directly involved in RNA 

 than in protein synthesis. 



Something should now be said about the very interesting in vitro 

 experiments of Allfrey, Mirsky and their colleagues on protein syn- 

 thesis in isolated thymus nuclei. These investigators found (1955, 

 1957) that if these nuclei are placed in a suitable medium, they 

 actively incorporate amino acids into their proteins, especially in 

 a protein fraction which is closely associated with DNA (Fig. 40, 

 p. 123). Treatment of the nuclei with deoxyribonuclease breaks 

 down part of the DNA and strongly inhibits the incorporation; 

 but ribonuclease has no effect on this system. According to Allfrey 

 and Mirsky (1957, 1958), the incorporation of amino acids in the 

 proteins of nuclei partially depleted of their DNA content can be 

 restored by the addition of unspecific DNA, of RNA and even of 

 synthetic polynucleotides. The explanation of these puzzling results 

 is that the incorporation process requires energy produced by phos- 

 phorylations in the nuclei themselves; these phosphorylations are 

 inhibited when DNA is removed from the nuclei and are restored 

 by the addition of polynucleotides. Finally, experiments in which 

 various inhibitors were used showed that a synthesis of RNA in the 

 isolated nuclei must precede the incorporation of the amino acids 

 into the proteins. One can conclude from this work of Allfrey and 

 Mirsky (1957, 1958) that the role of DNA in the synthesis of nu- 

 clear proteins is indirect rather than direct and that, as usual, RNA 

 involved in this synthesis. 



