296 NUCLEIC ACIDS AND GROWTH 3 



sors (amino acids, orotic acid) into their proteins and RNA for several weeks. 

 Their activity is normal during two weeks; afterwards, it begins to decrease and 

 becomes inferior to that of the nucleated pieces. More important still is the fact 

 that net synthesis of RNA and proteins occurs in the non-nucleated halves ; as a matter of fact, 

 this synthesis is even faster than in the nucleated fragments during the first 10-15 

 days following the operation. However, synthesis steadily decreases after this period. 



This finding is important because two popular hypotheses can now be ruled out: 

 first, it is obvious that since independent RNA synthesis can occur in the cytoplasm, 

 nuclear RNA is not necessarily a precursor of cytoplasmic RNA. Secondly, the 

 fact that growth, RNA synthesis and protein synthesis occur in non-nucleated 

 cytoplasm demonstrates that DNA is not directly concerned with these processes 

 (no DNA could be detected in the non-nucleated pieces, even when using an isotope 

 dilution technique). 



The case o^ Acetabularia clearly shows that Gale and Folkes' explanation for the 

 role of DNA in protein synthesis is not entirely correct: DNA can obviously not 

 act as an organizer for the synthesis of RNA in non-nucleated Acetabularia frag- 

 ments, since they contain no DNA. Nevertheless RNA and protein synthesis are 

 very extensive. However, the possibility remains that the slowing down of growth, 

 RNA and protein synthesis, which occurs when the fragments have been separated 

 from the nucleus for more than 10 days, is in some way linked to DNA. A compa- 

 rable situation to that just described for Acetabularia is found in the reticulocytes, 

 i.e. immature red blood cells which have lost their nuclei, but retained some of 

 their RNA store. As shown by Borsook et al. (1952), HoUoway and Ripley (1952), 

 Koritz and Chantrenne (1954), reticulocytes still incorporate amino acids into 

 their proteins, as well as glycine into their RNA (Kruh and Borsook, (1955). They 

 are even capable of synthesizing hemoglobin (Nizet and Lambert, 1953) and 

 various enzymes (Koritz and Chantrenne, 1954). During the maturation of the 

 reticulocytes, their RNA content steadily drops and an excellent correlation be- 

 tween RNA content and ability to incorporate amino-acids into the proteins has 

 been found (Holloway and Ripley, 1952, Gavosto and Rechenmann, 1954). 



Removal of the nucleus, in unfertilized eggs of the sea urchin (Malkin, 1954) 

 and the newt (Tiedemann and Tiedemann, 1954) has no measurable effect on the 

 incorporation of precursors into RNA and proteins. Net protein synthesis probably 

 does not occur in unfertilized eggs: but, at any rate, the experiments demonstrate 

 that the turnover of RNA and proteins does not require the presence of the nucleus 

 in egg cells. However, it should be noted that the experiments of Malkin (1954) 

 and Tiedemann and Tiedemann (1954) do not necessarily rule out an intervention 

 of DNA in the non-nucleated egg fragments, in view of the possible existence of a 

 DNA store in the cytoplasm of unfertilized eggs (see section v, p. 286). 



The only organism which has protein metabolism under close nuclear control 

 is Amoeba proteus. According to Mazia and Prescott (1955), removal of the 

 nucleus produces a six-fold decrease of the incorporation of methionine into the 

 proteins, within only a few hours. After 2-3 days, non-nucleated halves are 20 times 

 less efficient in this respect than nucleated fragments. Our own observations 

 (Brachet, i955d) on the same material, but with phenylalanine as a precursor, 

 indicate a similar trend. But the differences are much smaller than in Mazia and 



