i88 



300 



SUBCELLULAR PARTICLES 



200 



lOO 



Fig. I . Time course of incorpo- 

 ration of alanine- 1 -C", glycinc-i- 

 C" and lysine-2-C" into the pro- 

 teins of isolated thymus nuclei. In 

 this plot the data have been 'nor- 

 malized' to show the relative up- 

 takes for equivalent amounts of the 

 different amino acids, when each 

 has the same specific activity in 

 millicuries per millimol. 



30 60 90 



Time (minutes) 



J20 



ethanol, ethanol-ether-chloroform mixtures, ether and acetone. The protein residue 

 was then homogenized in acetone, plated on fiker paper planchets, and counted. 

 Radioactivity measurements were made using a thin-window gas flow counter 

 and scaling circuit, and the readings were subsequently corrected for self-absorp- 

 tion. Evidence that the uptake represents peptide bond formation and is not due 

 to adsorption or ester linkage has been given previously (6). 



The uptake curves obtained with several amino acids show a characteristic 

 'lag' phase of 5-15 minutes, followed by a rapid incorporation of label into the 

 proteins of the nucleus. After about 90 minutes, the rate of uptake falls of? as 

 some of the nuclei begin to autolyze and lose DNA. 



Although these results represent an average incorporation for all the nuclei 

 (plus the few cells) in suspension, the uptake in single nuclei can be followed by 

 radioautography; see (11) and (6). 



The nature of the lag" in the uptake of amino acids into the proteins of the 

 nucleus was investigated by experiments which are summarized in figure 2. 

 Nuclei were preincubated at 37° for 10, 20, or 30 minutes before adding the 

 alanine-i-C*. The time course of C ' incorporation by such preincubated nuclei 

 was compared with that of 'control' nuclei which had received the isotope at time 

 zero. It is clear that preincubation diminishes the lag period without affecting the 

 subsequent rate of C* alanine uptake. These findings suggest that an activation 

 of some sort occurs during preincubation. There is now evidence that this activa- 

 tion is synchronous with RNA synthesis in the nucleus (6). 



There are a number of experiments which show that the uptake of amino acids 

 is related to luiclcar protein synthesis. It was found that only the L-ainino acid 

 isomers are taken up (6) and that an amino acid once incorporated into protein 

 is not in rapid exchange with free amino acids in the medium (6). 



