JEAN BRACHET 



the work of Beth (1953) that the stem of the alga can grow with a limited supply of 

 light, while the formation of the umbrella definitely requires light of high intensity. 

 The preferential utilization o r one or other of the two mechanisms for protein 

 synthesis might thus have far-reaching morphogenetic consequences. 



Finally, it has also been shown (Brachet and Chantrenne, 1953; Chantrenne, 

 Brachet and Brygier, 1953) that, in certain batches of algae, the non-nucleated 

 fragments can react to the addition of io -3 m H 2 2 in the medium by increased 

 catalase activity; whether we are dealing with a real adaptative enzymatic synthesis 

 is not yet certain, but there is no doubt that after 2-3 weeks the capacity for 

 increased catalase activity on the addition of H 2 2 steadily decreases in the non- 

 nucleated halves. Increased catalase activity, although quantitatively much re- 

 duced, is still found when 3-month-old nucleated pieces are placed in sea water con- 

 taining H 2 2 . 



These experiments show clearly that in Acetabularia the nuclear control of protein 

 synthesis is a remote one; it is difficult to reconcile these findings with Caspersson's 

 ( 1 941) view that the nucleus is the main centre of protein synthesis. The experimental 

 results we got on Acetabularia are in good agreement with those obtained with the 

 basophilic non-nucleated red blood cells (reticulocytes) : these also incorporate 

 labelled amino-acids to a large extent into their proteins, for instance hemoglobin 

 (London et al., 1950; Holloway and Ripley, 1952; Chantrenne and Koritz, 1954; 

 Nizet and Lambert, 1953; etc.). In both cases (Szafarz and Brachet, 1954, for Aceta- 

 bularia; Holloway and Ripley, 1952, for reticulocytes) the decrease in protein syn- 

 thesis and RNA content run parallel, in accordance with our hypothesis (1945) that 

 microsomes play an essential part in protein synthesis. 



It should, however, be pointed out that these results do not exclude the possibility 

 that in the normal cell considerable protein synthesis may occur in the nucleus: 

 as already pointed out earlier in this paper, recent observations by A. Ficq (1953, 

 : 954) wno used an autoradiographic method, have demonstrated considerable 

 incorporation of radioactive glycine both in the proteins and in the ribonucleic acid 

 of the nuclei in various embryonic and adult tissues. In the case of starfish oocytes, 

 incorporation of glycine is very much higher in the nucleolus than in the nuclear sap 

 or cytoplasm. 



CONCLUSIONS 



As most of the experimental results reported in this paper have already been thor- 

 oughly discussed, the conclusions are brief. It is now obvious that the nucleus is 

 neither the centre of cellular oxidations nor a storehouse or building place for all 

 enzymes; it might, however, be concerned with the synthesis of nucleotides, whether 

 nucleic acids or coenzymes, and in this way might regulate many of the biochemical 

 activities of the cell. To consider that the nucleus is the main centre of protein syn- 

 thesis in the cell is also an exaggeration, since extensive building up of proteins can 

 still go on in its absence; this does not mean that in a normal cell, especially a grow- 

 ing one, important protein syntheses cannot occur also in the nucleus, particularly 

 in the nucleolus. 



Our knowledge of the chemical changes undergone by the cell when its nucleus 

 has been removed is still too incomplete for drawing far-reaching conclusions : but it 



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