STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 93 



uptake has been found to be most rapid in the rhizoid and in the nucleolus. 

 Enucleate fragments also maintain their ATP content aerobically and 

 anaerobically. Active protein and RNA synthesis occur simultaneously in 

 enucleate fragments; indeed, over a period of three weeks protein synthesis 

 is much more active in these than in the nucleate fragments, although it then 

 stops in the former while the latter continues to synthesize protein. With the 

 exception of this pomt, no evidence has been obtained in this organism to 

 indicate that protein synthesis by the nucleus is quantitatively significant. 



With Amoeba proteus, somewhat different results are obtained (Brachet, 

 1955a). Enucleation results in little change of O2 consumption bii.t ATP 

 accumulates in air. Anaerobically the ATP content of the enucleate fragment 

 falls quickly, suggesting that in this organism the nucleus may play a 

 quantitatively significant role in the anaerobic generation of ATP. Unhke 

 Acetabularia, enucleate amoebae shrink and their glycogen, RNA, and pro- 

 tein contents faU markedly after a very few days. In the nucleate half, P^^ 

 uptake is maintained at a normal rate, whereas this falls sharply in the 

 enucleate fragment.^ 



Enzyme studies have proven more difficult in Amoeba. Many enzymes 

 have not yet been found in this organism, e.g., hexokinase, DPN pyrophos- 

 phorylase, nucleoside phosphorylase. Enolase, beheved to be concentrated 

 in the nuclei of other cells, is not so concentrated in Amoeba and may be 

 maintained unchanged for many days in an enucleate fragment. On the other 

 hand, dipeptidase, phosphatase, and esterase fall markedly in enucleate 

 fragments; the last two are known to be associated with the microsomal 

 fraction. The results on RNA and enzyme disappearance suggest that, in 

 this organism, the nucleus appears to control the maintenance of the micro- 

 somal fraction in the cytoplasm, a fact which parallels the absence of such a 

 fraction in enucleate mammahan erythrocytes. Such control apparently does 

 not apply to other enzymes, such as enolase, protease, amylase, and ATPase, 

 which may be associated with other cell fractions. 



More direct data on protein synthesis in amoebae stem from the work of 

 Mazia and Prescott (1955). Enucleation reduces immediate total methio- 

 nine-S^^ uptake by a factor of 2.4; indeed, the rate of incorporation into 

 protein in enucleate halves was only one-sixth that in nucleate halves. The 

 latter effect accounts for the observed effect on uptake, i.e., the nucleus does 

 not control penetration into the cell, but only incorporation into protein. In 

 this system, nuclear synthesis is estimated to account for 40 % of the 

 methionine incorporation into the whole cell. On the other hand, intact 



^ The marked decrease of anaerobically-generated ATP has also been observed hi two 

 other systems ui which nuclei have been damaged, i.e., ultraviolet-irradiated E. coli 

 (Kanazir and Errera, 1955) and strongly lethal interspecific frog hybrids, as well as in 

 frog eggs fertilized with sperm treated with nitrogen mustard (Brachet, 1954). 



