NUCLEAR PROTEIN SYNTHESIS 



275 



was surprising therefore, to find that the addition of isolated thymus DNA 

 to the ribosome suspension stimulates the uptake of [i -^^C]-leucine into ribo- 

 somal proteins. As shown in Fig. 8, the increase in specific activity of the 

 protein is appreciable; DNA-supplementedribosomes are 50% more active 

 than the corresponding "controls". The figure also shows that the 

 simultaneous addition of DNAase with the DNA abolishes the stimu- 

 latory effect [44]. 



There is, as yet, no evidence for the tissue- or species-specificity of 

 DNA in promoting uptake. DNA preparations from calf thymus, calf 



xDNA lOmg/ml. 



Control 



X 



xDNA lOmg/ml. 

 DNAase 05 mg/ml 



PES 10 mg/ml. 



I I I I ] I I 



20 40 60 



Time (mln) 



Fig. 8. Effects of added DNA, DNA + deoxyribonuclease, and polyethylene 

 sulphonate on [i-^*C]-leucine incorporation into the proteins of isolated nuclear 

 ribosomes. The specific activity of the ribosomal protein (in c.p.m./mg.) is plotted 

 against the time of incubation at 37°. 



kidney, and wheat germ are all equally efi"ective in this test system. On the 

 other hand, the results cannot be explained as simply a matter of adding 

 large negatively-charged molecules (which could bind traces of ribonu- 

 clease in the preparations and so enhance their activity) because the 

 addition of an equal amount of polyethylene sulphonate actually inhibits 

 amino acid uptake by isolated nuclear ribosomes (Fig. 8). 



The stimulatory efiPect of DNA takes on added interest in view of 

 related findings that added histones inhibit amino acid uptake by nuclear 

 ribosomes [44] (Fig. 9). The role of histones as repressors of nuclear 

 synthetic activities was observed earlier in studies of amino acid uptake in 

 intact nuclei (Fig. 10) [49]. 



The stimulation of protein synthesis in the ribosomes by DNA and 

 the corresponding inhibition by histones suggest that the synthetic activity 

 of ribosomes within the nucleus will vary depending on their proximity to 



