NUCLEAR PROTEIN SYNTHESIS 277 



Summary 



A study of isolated thymus nuclei and sub-nuclear fractions shows that 

 the pathway of protein synthesis in the nucleus involves a sequence of 

 reactions similar to that observed in cytoplasmic systems. 



The nucleus contains amino-acid-activating enzymes which act on at 

 least fifteen L-amino acids ; D-amino acids are not activated. 



Following activation, the amino acid is transferred to a nuclear RNA 

 fraction. Neither DNA nor the high molecular weight RNA of the nucleolus 

 takes part in this process. 



Analysis of the nuclear leucyl-RNA complex shows that the receptor 

 group in nuclear "carrier" RNA is a terminal adenylic acid. 



Ribonucleoprotein particles in the nucleus are active sites of protein 

 synthesis. These particles (termed nuclear ribosomes) can be isolated and 

 fractionated by differential ultracentrifugation of nuclear extracts, and are 

 capable of independent amino acid incorporation, provided they are 

 isolated and tested under isotonic conditions. 



Amino acid uptake into the isolated nuclear ribosomes requires the 

 presence of ATP, amino-acid-activating enzymes, and GTP. 



The addition of DNx^ promotes amino acid uptake in ribosome sus- 

 pensions while added histones inhibit the process. This may prove to be 

 one of the physiological mechanisms for the direction and control of 

 nuclear protein synthesis. 



The protein synthetic process in isolated nuclei responds to the addition 

 of sodium ions because Na + is required in a specific amino acid transport 

 mechanism. This transport of amino acids to the sites of nuclear protein 

 synthesis is apparently an enzymic reaction that is specific for the L-form 

 of the amino acid. 



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