NUCLEAR PROTEIN SYNTHESIS 269 



ditions (upper curve) ; ribonucleoprotein particles isolated in, or exposed 

 to hypertonic sucrose solutions are completely inert (lower curve). This 

 finding is in agreement with earlier experiments on intact nuclei showing 

 that amino acid uptake proceeds best in isotonic sucrose solutions and is 

 impaired when nuclei are exposed to either hypertonic or hypotonic 

 conditions [44]. 



Leucine-l-C , , 



Isotonic nuclear 



nbosomes 



Ribosomes after brief 

 hypertonic exposure 



20 40 60 

 Incubation time (min) 



Fig. 3. The time course of incorporation of [i-^*C]-leucine into isolated 

 ribonucleoprotein particles prepared from thymus nuclei. The incubation medium 

 contained (in addition to the particles), nuclear pH 5 enzymes. ATP + an ATP- 

 regenerating system, and GTP. The upper curve shows the specific activity of 

 the proteins of the ribosomes under isotonic conditions. A brief exposure to 

 hypertonic sucrose solutions destroys the capacity for amino acid uptake under 

 these conditions (lower curve). 



The incorporation of amino acids by isolated nuclear ribosomes is 

 inhibited if ribonuclease is added to the incubation medium. This eiTect 

 is of interest because it contrasts with findings on intact nuclei. It has been 

 shown that ribonuclease does not affect amino acid uptake into the ribo- 

 somes of the intact isolated nucleus [37]. The results indicate that the RNA 

 of the ribosome fraction is immune to attack by RNAase as long as it 

 remains inside the nucleus. This is one of the unique properties of the 

 protein synthetic pathway in the nucleus. 



Some characteristically nuclear aspects of amino acid 

 incorporation 



On the whole, the study of sub-nuclear fractions has shown that the 

 protein synthetic mechanism in the cell nucleus is very much like that of 



