118 ROLE OF THE CELL NUCLEUS 



tion in Acetabularia remains unclear. There is some evidence, how- 

 ever, that sulfur metabolism must be involved in this morphogen- 

 etic process. It was found that treatment of the algae with mercapto- 

 ethanol, a sulfhydryl-containing substance, completely inhibits 

 cap formation. On the other hand, dithiodiglycol (which is the 

 oxidized counterpart of mercaptoethanol and which thus contains 

 a disulfide linkage) stimulates the formation of caps in enucleate 

 stalks. At very low concentrations, p-chloromercuribenzoate, which 

 is also a sulf hydryl inhibitor, also exerts a stimulatory effect on cap 

 production. Finally, methionine and, to a lesser extent, ethionine 

 also have a favourable effect on the formation of caps in enucleate 

 fragments (Brachet, 1959.) It is thus likely that the formation or 

 absence of a cap is linked to some enzymatic system, the activity of 

 which is regulated by the sulf hydryl-disulfide equilibrium. It would 

 be of interest to study this equilibrium under the conditions of illu- 

 mination used by Beth (1953a, b; 1955) in order to modify the size 

 of the caps. 



In order to get a better insight into the mechanisms of protein 

 synthesis in Acetabularia, Brachet et al. (1955) studied the incorpo- 

 ration of ^^COa in the proteins of nucleate and enucleate halves. It 

 was found that the incorporation reaction proceeds at the same 

 rate in both fragments for two weeks. At that time, incorporation 

 becomes progressively less active in the enucleate halves than in the 

 others ; after 7 weeks, the incorporation in the proteins of enucleate 

 stalks becomes 2.4 times less than in the nucleate rhizoids. 



It can be concluded that the incorporation experiments entirely 

 confirm the results obtained for net protein synthesis. They further 

 show that, even when net protein synthesis has ceased in the enu- 

 cleate pieces, protein turnover continues for several weeks in the 

 absence of the nucleus. 



The incorporation of ^^COg into the proteins of Acetabularia, as 

 one would expect, requires light; it becomes negligible in the dark. 

 Further indications that the process is closely linked to photosyn- 

 thesis are found in the fact that the specific activity of chloroplastic 

 proteins is 2 to 3 times higher than that of the other proteins. 



A different situation is found when ^^C-glycine is used as a pre- 



