52 THE BIOSYNTHESIS OF PROTEINS 



1954; Brachet etal., 1955; Clauss, 1958). This means that in Acetahularia a. 

 complete system for making protein material is contained in the cytoplasm 

 and can operate in the absence of the nucleus. 



Studies on non-nucleate fragments of sea urchin eggs (Malkin, 1954) 

 or of newt eggs (Tiedemann and Tiedemann, 1954) also showed that 

 various labelled protein precursors can be incorporated into proteins in a 

 cytoplasm without a nucleus. Rabbit reticulocytes is another example of an 

 enucleate cell which can incorporate labelled amino acids into its proteins 

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

 Nizet and Lambert, 1953). 



However, in all the experiments mentioned above there was no evidence 

 that the protein material synthesized in the absence of the nucleus was 

 made of normal protein. Some data on reticulocytes made it probable that 

 regular haemoglobin was made in these enucleate cells (Hammarsten et al., 

 1953). But this important question has been answered only recently in 

 studies on the net synthesis of specific enzymes in fragments of Aceta- 

 hularia. Thus enolase is made in the absence of the nucleus at the same rate 

 as total protein material for at least eleven days after enucleation (Baltus, 

 1959) and net synthesis of phosphorylase and of invertase goes on at an 

 almost normal rate for two or three weeks in enucleate cells (Clauss, 1959). 

 This means that cytoplasmic fragments of Acetahularia contain a perfect afid 

 complete system for making certain proteins, including the genetic information 

 required for the synthesis of active enzymes. 



This conclusion must, however, be qualified. Protein synthesis is inde- 

 pendent of the nucleus /or some time only: a change occurs in the non- 

 nucleate fragments about two weeks after the algae have been cut. Between 

 the twelfth and the fifteenth days after section, the rate of incorporation of 

 ^'*C02 into the non-nucleate fragments drops to about 30 per cent below 

 that of the nucleate halves, and net synthesis of proteins stops altogether, 

 although the enucleate fragments can survive for at least two months there- 

 after (Brachet and Chantrenne, 1951 ; Vanderhaeghe, 1954; Brachet et al., 

 1955; Richter, 1959). Protein synthesis in older algae is more independent 

 of the nucleus than in younger ones (Hammerling, 1956). It would appear 

 that some substance produced by the nucleus is required for maintaining 

 protein synthesis and that this substance is slowly exhausted when the 

 cytoplasm is deprived of the nucleus. There is, however, no evidence that 

 this hypothetic substance is a specific gene product: it might be quite a 

 common biochemical originating in the nucleus (Brachet, 1952, 1954; 

 Brachet and Chantrenne, 1956). 



Moreover, in Acetahularia as well as in Amoeha, the formation of certain 

 proteins, e.g. acid phosphatase (Keck and Clauss, 1958) appears to depend 

 more directly on the presence of the nucleus; for it stops shortly after 

 enucleation, i.e. at a time when the bulk of cytoplasmic proteins and 



