EXPERIMENTAL RESULTS 119 



cursor; its incorporation has, unfortunately, not been studied in 

 fragments. It is interesting that the glycine uptake and incorpora- 

 tion are not dependent on the presence or absence of light. In con- 

 trast to the results obtained with ^^COa, the incorporation is stronger 

 in the microsomes than in the chloroplast fraction. A comparison 

 of the results obtained with ^^COa and with glycine suggests that 

 Acetabularia possesses two biochemically different mechanisms for 

 protein synthesis. One of them requires CO2, light and chloroplasts ; 

 in the second, where an amino acid is the precursor, microsomes 

 are more important than chloroplasts. Such a conclusion stands in 

 perfect agreement with the work of 5er/z (1953b, 1955) who demon- 

 strated that growth of the stalks requires less light than cap formation. 



A few more observations pertaining to the protein metabohsm in 

 Acetabularia deserve mention. For instance, incorporation of ^^C- 

 glycine into the proteins is 45 % inhibited when the whole algae are 

 placed for 2 weeks in the dark. Another significant fact is that treat- 

 ment of the algae with 10-^ M thiouracil— which acts as an inhibitor 

 of RNA metabolism— also inhibits (30%) the incorporation of 

 glycine into the proteins of whole algae. These observations strongly 

 suggest that RNA is involved, as usual, in the protein metabolism 

 of Acetabularia. This conclusion is shared by Werz (1957), who 

 studied the effects of trypaflavine on regeneration in Acetabularia. 



We have seen that in Amoeba different enzymes are unequally 

 placed under nuclear control; what is the equivalent situation in 

 Acetabularia'^ It is known (Brachet et ah, 1955) that, as in Amoeba, 

 the enzymes which play a part in nucleotide metabolism and which 

 are said to be accumulated in the liver nuclei (DPN-synthesizing 

 enzyme, adenosine deaminase, nucleoside phosphorylase, guanase) 

 are in concentrations too low to be detected. But it is interesting 

 that aldolase is synthesized (40% increase) in the regenerating 

 enucleate stalks at much the same rate as the total proteins (Baltus, 

 1959). In agreement with Stern and Mirsky's (1952) observations on 

 isolated wheat germ nuclei, aldolase is concentrated in the nucleate 

 half. However, it is not dependent on the presence of the nucleus for 

 its synthesis in Acetabularia. 



Recently, Hammerling and his co-workers (1959) have studied a 



References p. 133/135 



