270 VINCENT G. ALLFREY 



the cytoplasm. It involves (i) amino acid activation, (2) transfer of the 

 amino acid to the terminal adenylic acid of a carrier RNA, and (3) in- 

 corporation into the proteins of the ribosome fraction by a reaction sequence 

 requiring GTP. Yet there are some striking differences between nuclear 

 and cytoplasmic incorporation systems. The first of these is the specific 

 requirement of the nucleus for sodium ions. 



Why is protein synthesis in the nucleus sensitive to the presence or 

 absence of sodium ions ? This question has been studied in suspensions of 

 isolated cell nuclei by tracer methods, and the answer is now clear. Sodium 

 ion dependency reflects the operation of a transport system for bringing 

 amino acids into the cell nucleus. 



NUCLEAR AMINO ACID TRANSPORT 



The effect of sodium ions on the overall process of amino acid in- 

 corporation into nuclear proteins was discovered (largely by accident) at 



200 



Alanine-1-C 



■ ^^ K_* Cs* and Rb* 



002 0-04 006 008 

 Salt concentration (M) 



Fig. 4. The effect of adding different monovalent cations (as chlorides) on the 

 incorporation of [i-"C] alanine into the proteins of intact isolated thymus nuclei. 

 The specific activity of the nuclear protein after 60 min. incubation is plotted 

 against the salt concentration of the medium. 



the very beginnings of our study of uptake processes in the nucleus [5, 

 45]. The magnitude of the effect is strikingly demonstrated in Fig. 4, 

 which compares the uptake of [i-^^C]-alanine in a medium containing 

 sodium ions with that observed in the presence of equivalent amounts of 

 potassium (or other monovalent cations). The stimulatory effect of added 



