50 THE BIOSYNTHESIS OF PROTEINS 



enucleation. This clearly indicates that the cytoplasm contains a complete 

 system for incorporating amino acids into protein material and that the 

 pathway for this residual incorporation does not go through the 

 nucleus. 



The drop in incorporating activity after enucleation might be due to 

 indirect effects, irrelevant to a nuclear control of cytoplasmic protein 

 synthesis, for many changes occur in the general metabolism of the Amoeba 

 as a result of enucleation. The enucleate part looses, for instance, the 

 capacity to use its lipid and glycogen stores, and it is heavily handicapped 

 as against nucleate parts for so basic a function as glycolysis (Brachet, 1955, 

 1956, 1957; Brachet and Chantrenne, 1956; Chantrenne, 1958). On the 

 other hand, the fact that the maintenance of various proteins is not equally 

 dependent on the presence of the nucleus makes one suspect that the 

 cytoplasm can make proteins but that a considerable fraction of the 

 proteins normally present in the cytoplasm of the amoeba are made under 

 very close nuclear control and possibly within the nucleus itself. 



Another material which is very well suited for merotomy experiments is 

 the green marine alga Acetahularia mediterranea, which belongs to the class 

 of Dasycladaceae. During most of its life, the alga consists of a rather thick 

 filament or stalk attached to the rocks at one extremity by rhizoids. The 

 stalk can be 1-2 in. long ; it is made of one single cell, the nucleus of which is 

 always located in one of the rhizoids. At the end of the life cycle, the tip of 

 the stalk develops into an umbrella. When this is fully grown, the big 

 nucleus divides into many small nuclei which invade the cytoplasm and 

 finally reach the umbrella where they become enclosed in the cysts. These 

 are the resistant form of the organism ; each contains a few nuclei. After 

 a maturation period, the cysts can germinate and liberate flagellate gametes. 

 Conjugation of two gametes gives a uninucleate zygote. This grows into 

 rhizoids and a stalk, which will later develop an umbrella (see e.g. Kiihn, 

 1955). The complete cycle takes one year in nature and about six months 

 in the laboratory. 



During the part of the life cycle which extends from the formation of the 

 zygote up to the complete development of the umbrella, Acetahularia is a 

 single cell ; the nucleus is located in a rhizoid and the cytoplasm is uniformly 

 filled with chloroplasts. The cell is surrounded by a tough polysaccharide 

 casing. If the alga is cut across the stem, both fragments recover very well 

 and can be kept alive for more than two months. The fragment with the 

 rhizoids contains the nucleus, the other one is purely cytoplasmic. Both 

 fragments are amply provided with chloroplasts, and photosynthesis is not 

 impaired by enucleation (Brachet et al., 1955). Extremely interesting ex- 

 periments were performed on Acetahularia by Hammerling (1934, 1946, 

 1953) and by his collaborator Beth (1943). If the nucleate part of one 

 strain is grafted on an enucleate part of another strain of Acetahularia, the 



