10 J. Bracket 



to a value of 6 after a few hours, and it reaches the value of 

 20 at the end of 2-3 days. We must bear in mind, however, 

 that methionine uptake by the proteins of non-nueleated 

 halves never drops to zero. Such uptake is strongly dimin- 

 ished, but never abolished by removal of the nucleus. 



More recently, in experiments done with our co-worker 

 Mrs. A. Ficq, in which radioactive phenylalanine was used as 

 a precursor and located in the cell by autoradiography, we 

 have essentially confirmed Mazia and Prescott's results. The 

 differences we observed are less striking, however, since our 

 N/A ratios are in the neighbourhood of 2 (instead of 6-20) from 

 1 to 6 days after removal of the nucleus in the amoeba and in 

 the neighbourhood of 5 after 10 days. This is taken to mean 

 that, in the amoeba, removal of the nucleus does not immedi- 

 ately stop protein metabolism in the cytoplasm. As a matter 

 of fact, the amino acid uptake by proteins of the non-nucleated 

 halves begins markedly to decrease only when the RNA 

 content of the non-nucleated cytoplasm is already much 

 diminished (Linet and Brachet, 1951). 



Thus we come to the notion that, in the case of the amoeba 

 at least, the nucleus cannot be the exclusive centre of protein 

 synthesis. Amino acid incorporation into proteins is main- 

 tained at a non-negligible rate in non-nucleated fragments as 

 long as the RNA content of the latter remains essentially 

 unchanged. The same experiments, on the other hand, show 

 very clearly that the nucleus exerts a control on protein 

 metabolism in the cytoplasm. This gives rise to another 

 problem: are all the cytoplasmic proteins equally dependent 

 on the nucleus? 



This was investigated, still using amoebae, by following in 

 the course of time the changes of various enzymes (hence of 

 as many distinct proteins) in both types of fragments. The 

 results were essentially as follows (Brachet, 1955a). In the 

 amoeba, the removal of the nucleus results in widely different 

 effects in the case of different enzymes. Some enzymes, such 

 as protease, amylase and enolase, remain practically un- 

 changed after removal of the nucleus ; others, dipeptidase for 



