92 S. S. COHEN 



proteins of rat pituitary was most rapid in the soluble fraction and nuclear 

 incorporation occurred at less than half this rate (Ziegler and Melchior, 1956). 

 Incorporation of leucine-C^* into TMV-uifected leaf disks over a two-hour 

 period revealed that TMV had a lower specific activity than any of the 

 isolated fractions, including chloroplasts; the microsomal fraction was most 

 active, more than twice that of the isolated TMV (Stephenson et at., 1956). 

 In this study it was also found that isolated chloroplasts synthesize protein 

 quite actively, and that oxygen and light stimulated this process. 



In studies of Allfrey et al. (1955a) on the incorporation of glycine-N^^ into 

 various mouse tissues, special attention was paid to the activity of nuclear 

 components. In general, cytoplasmic ribonucleoprotein was most active in 

 these tissues and the residual protein of the chromosomes approached the 

 over-aU rate of cytoplasm. However, glycine-N^^ uptake into histones was 

 least rajiid of any protein fraction, including the Hpoproteins and globulins 

 of nuclei (Rotherham et al., 1957). Although protein synthesis has been 

 described in isolated nuclei, the quantitative role of the nucleus in the 

 over-all protein economy of the cell has not been greatly clarified by the 

 experiments indicated above. 



a. Enucleate and Nucleate Cell Fragments. Brachet and his collaborators 

 have studied fragments of the unicellular alga, Acetabularia mediterranea, and 

 the protozoan. Amoeba proteus. Mazia and his group have concentrated on 

 the latter organism. The experiments of these groups have attempted to 

 clarify the nature of the interactions between nucleus and cytoplasm that 

 occur in the metabolizing cell. The procedure adopted is essentially one of 

 study of differences in metaboUc behavior when the possibihty of these 

 interactions is eliminated in one of the fragments. Studies of these systems 

 have yielded considerable fundamental information. 



Although virologists are familiar with amoebae, probably few are 

 acquainted with Acetabularia, an organism studied in detail by Hammerling 

 (1953). The fully developed giant cell consists of a rhizoid, stalk, and cap. 

 The stalk may reach 4 to 6 cm., the cap diameter being up to 1 cm. The single 

 large nucleus lies within the rhizoid and removal of the rhizoid produces an 

 enucleate part devoid of DNA. The cap may also be cut from this enucleate, 

 chloroplast-containing fragment. The cap may be regenerated in the light, 

 photosynthesis permitting the fixation of C^^Og into cell components. Even 

 after 5 weeks, the synthetic activity of enucleate parts may be as much as 

 70 % that of nucleate fragments. It may be mentioned that the nucleolus of 

 the intact organism is rich in RNA, as indeed is the cytoplasm of the rhizoid 

 in the vicinity of the nucleus. 



According to Brachet et al. (1955), removal of the nucleus has no effect on 

 Og consumption or photosynthetic activity for two more weeks, and little on 

 the incorporation of P^^ for shorter intervals, e.g., hours. However, P^^ 



