EXPERIMENTAL RESULTS 99 



nucleolar RNA is a precursor of cytoplasmic RNA, but they did 

 not prove the point directly. 



However, recent observations of Woods and Taylor (1959), who 

 used tritium-labeled uridine as a specific precursor, are entirely 

 compatible with the view that RNA is synthesized primarily in the 

 nucleus and that cytoplasmic RNA is largely, if not entirely, of 

 nuclear origin; they observed that in plant cells the nuclei are 

 the first to become radioactive. If, at that time, the cells are trans- 

 ferred to a non-radioactive medium, the labeled nuclear RNA can 

 be seen to move progressively toward the cytoplasm. Similar obser- 

 vations have been made by Zalokar (1959) in the case of centrifuged 

 Neurospora cells. 



Let us now consider protein metabolism. In starfish and am- 

 phibian oocytes, nucleoli incorporate precursors (glycine or phenyl- 

 alanine) at a faster rate than does either nuclear sap or cytoplasm, 

 according to Ficq (1955a, b). But the differences are never as 

 marked as in the case of the incorporation of purines or pyrimidines 

 into RNA. The same remark can be made for centrifuged amphib- 

 ian oocytes. While the incorporation of adenine is, as we have just 

 seen, very intense in the loops of the lampbrush chromosomes, the 

 differences between the loops, the rest of the nuclear sap and the 

 cytoplasm is much less conspicuous for proteins. In amoebae and 

 in Acetabularia, the nucleus seems to be somewhat more active than 

 the cytoplasm as regards the incorporation of amino acids into 

 proteins ; but again, a difference between the nucleus and the cyto- 

 plasm is never as strong as it is for RNA anabolism. In higher or- 

 ganisms, liver cells show a much faster incorporation of amino acids 

 into nuclear proteins than into cytoplasmic proteins (Ficq and 

 Errera, 1955; Moyson, 1955). However, liver cells are the exception 

 rather than the rule, since Ficq and Brachet (1956) did not find any 

 conspicuous accumulation of labeled phenylalanine in the nuclei 

 of pancreas, intestine, lung, heart, muscle, kidney, spleen and uterus 

 cells of the mouse. Labeling of nuclear proteins, methionine being 

 used as a precursor, has recently also been reported by Pelc (1956). 

 The nuclei show appreciable incorporation of the precursor even 

 in tissues where mitotic activity is very small (thyroid, seminal 



References p. 133/135 



