BIOCHEMICAL PROPHRTIUS OF THE ISOLATED NUCLEUS 



189 



100 



PREINCUBATION TIME 



ME IN MINUTES 



10 20 30 40 50 60 



Fic. 2. Effect of preincubatum of thyimis nuclei upon the subsequent incorporation of 

 alanine-i-C". The nuclear suspension was preincubated for 10, 20 or 1,0 minutes before adding 

 the isotopic alanine. The time course of C" incorporation by such preincubated nuclei is 

 compared with that of control nuclei which received the isotope at time zero. Note that pre- 

 incubation diminishes the lag period without affecting the subsequent rate of C"-alanine 

 incorporation. 



Other evidence relating amino acid uptake to protein synthesis is obtained when 

 one fractionates the proteins of the nucleus, following C^*-labeling experiments. 

 Several classes of proteins of differing activities can be prepared. A flow sheet sum- 

 marizing the fractionation is shown in figure 3. The distribution ol isotope in 

 these different fractions (table i) is of some interest because it shows that the 

 protein most closely associated with the deoxyribonucleic acid is more active than 

 all other proteins of the nucleus ( with the exception of a small fraction extractable 

 in pH 7.1 buffer). A second point of interest is the fact that the level of isotope 



