SITES WITHIN THE CELL 55 



4. Synthesis of Protein in the Cell Nucleus 



Cytochemical data, especially observations of amino acid incorporation 

 into cell protein by autoradiographic techniques, indicated that the nucleus 

 is not as active a centre of protein synthesis as the ergastoplasm. Neverthe- 

 less, labelled amino acids were found within a relatively short time in the 

 nuclear protein ; and in liver and lymph nodes, the in vivo incorporation of 

 labelled phenylalanine into the nucleus, as judged from radioautography 

 experiments, represents a considerable fraction of total incorporation 

 (Ficq and Brachet, 1956; Ficq, 1959; Ficq and Errera, 1959; Gavosto and 

 Ficq, 1954; Zalokar, 1960b). More recent observations (Zalokar, 1960a) 

 on Drosophila show that protein synthesis begins a little later in the 

 nucleus than in the cytoplasm. 



The drop in amino acid incorporation following enucleation of Amoeba 

 (Mazia and Prescott, 1955), the decrease of certain enzymes under the same 

 conditions (Brachet, 1955-1957) or the passage of ^53 labelled proteins 

 from a grafted labelled nucleus to the unlabelled nucleus (Goldstein, 1959) 

 could be interpreted as indications that certain proteins were made in the 

 nucleus. Daly et al. (1952) showed that certain nuclear proteins of various 

 mouse tissues incorporate labelled glycine in vivo at a rate comparable with 

 the average cytoplasmic proteins. Similar results were later obtained by 

 Smellie et al. (1953) for rat liver. It is not quite clear, however, whether the 

 labelled proteins found in the nuclei were made within the nuclei, or 

 whether they originated in the cytoplasm. 



AUfrey (1954) succeeded in isolating nuclei of thymus which are able to 

 incorporate amino acids in vitro into their proteins. Adequate preparations 

 can be obtained by fractional centrifugation of a calf thymus homogenate 

 in sucrose solutions. Allfrey et al. (1955, 1956, 1957) thoroughly studied 

 the requirements for the incorporation of labelled amino acids and estab- 

 lished convincingly that labelled alanine, glycine and lysine are incorpor- 

 ated to various extents into several typical nuclear proteins in isolated 

 thymus nuclei. Radioautography experiments (Ficq and Errera, 1958, 1959) 

 confirmed that the incorporation (of phenylalanine) really occurs in clean 

 nuclei and is not due to contamination with remnants of cytoplasm or with 

 thymocytes which are usually present in small numbers in the prepara- 

 tions. Since only the L-amino acid isomers are taken up, and since an 

 amino acid once incorporated into protein is not exchanged with free 

 amino acids in the medium, it can be taken as most probable that 

 the incorporation represents true synthesis of nuclear proteins (Allfrey 

 et al, 1957). 



Thymus nuclei were for several years the only type of nuclei with which 

 incorporation could be observed in vitro. Logan et al. (1959) recently 

 obtained similar results with isolated nuclei from rat liver. All these 

 observations make it clear that nuclei contain all that is necessary for 



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