NUCLEAR PROTEIN SYNTHESIS 265 



protein particles of the nucleus. The association of the activating enzymes 

 with particles which have been proven to be intranuclear (see below) is 

 additional evidence for the conclusion that the nucleus itself carries out 

 the amino acid activation reaction. 



AMINO ACID TRANSFER TO NUCLEAR RIBONUCLEIC ACID 



Studies of many cytoplasmic and bacterial systems have shown that 

 the sequel to amino acid activation is a transfer reaction in which the 



1500 



RNA (00671^ chloramphenicol) 



Protein (0067 M chloramphenicol) 

 i — n 



10 20 30 40 50 60 

 Time (min) 



Fig. 2. The time course of uptake of [i-^^C]-leucine into the protein and 

 " carrier" RNA of isolated thymus nuclei. The specific activity of the total nuclear 

 protein (c.p.m./mg.) is plotted in the upper curve. The incorporation into protein 

 is greatly inhibited by the presence of chloramphenicol (lower curve). The uptake 

 of amino acid into the RNA is the same in chloramphenicol-treated nuclei as in 

 "controls". 



activated amino acid is coupled to a soluble ribonucleic acid, according to 

 the equation : 



enzyme — amino acyl ~ AMP + sRNA^ 



amino acyl — sRNA + enzyme + AMP 



[24-28]. A similar process occurs in the isolated thymocyte nucleus [19, 29]. 

 The transfer of amino acid to RNA in the nucleus can be shown directly 

 by isolation of radioactive "carrier" RNA after incubating nuclei in the 

 presence of a radioactive amino acid and chloramphenicol [19] or puromy- 

 cin [29, 30]. The addition of either of these antibiotics blocks amino acid 

 incorporation into nuclear protein, but does not interfere with the forma- 

 tion of the amino acyl-RNA complex [29]. The RNA can then be prepared 

 by the phenol method [31, 32] without risk of contamination by radio- 

 active protein. The results of such an experiment are summarized in Fig. 2, 

 in which the specific activities of nuclear protein and nuclear "phenol" 



