80 S. S. COHEN 



than 50 %. In addition, a structural analog of a purine nucleoside, 5, 

 6-dicliloro-^- D-ribofuranosyl benzimidazole, whicli is reported to inliibit 

 influenza virus multiplication (Tamm et al., 1954), is also an effective 

 inhibitor of uptake. To be maximally inhibitory, a jS-D-ribofuranoside is 

 required (AUfrey et al., 1957b). 



These workers have also shown that amino acid incorporation is greatly 

 inhibited by degradation and removal of DNA after treatment with DNAase.^ 

 Readdition of large fragments of DNA to such DNAase-treated nuclei 

 restores activity. Bases and nucleotides were ineffective in restoring activity. 

 Although many degradation products of RNA were ineffective in this regard, 

 dialyzable split products of RNA digestion by RNAase had some restoring 

 activity. The synthetic polynucleotide, polyadenylic acid, possessed restoring 

 activity. The possible role of RNA metabohsm in incorporation in this 

 system is unclear at present; on the other hand, DNA is imphcated in protein 

 synthesis in nuclei. It may be noted that of the proteins containing incor- 

 porated amino acid the histones had the lowest specific activity, whereas 

 the proteins associated with RNA in "residual chromosomes" had the highest 

 specific activity. 



The synthesis of nucleic acid in these nuclei has been studied by estimating 

 the incorporation of glycine-1-C^* and orotic acid-6-C^* into purines and 

 pyrimidines, respectively (AUfrey et al., 1957a).^ The incorporation of the 

 latter was observed far more readily. It enters nuclear RNA, from which it 

 can be released by RNAase. Nuclei degraded by DNAase are far less effective 

 in such incorporation. The incorporation of thymidine into the DNA of 

 isolated nuclei had been demonstrated by Friedkin and Wood (1956), who 

 also observed that DNAase lowers incorporation. 



The benzimidazole riboside was shown by Allfrey et al. (1957a) to block 

 synthesis of nuclear RNA, using orotic acid-C^^ as a label. This compoimd 

 was capable of blocking protein synthesis when added with the amino acid. 

 However, a 30-minute incubation and activation of the nuclei before addition 

 of the analog protected the nuclei from inhibition of amino acid incorporation 

 in the presence of the analog. One might imagine that a prior synthesis of 

 nuclear ribonucleotides or RNA was essential to subsequent amino acid 

 incorporation which, having had its precondition fulfilled, could no longer be 

 blocked by the analog riboside. 



^ According to a number of workers (Brown et al., 1952; Lamirande et al., 1954), the 

 nucleases, DNAase and RNAase, can be found in nuclei. However, most of the nuclease 

 activity of the cell is associated with cytoplasmic structures, e.g., lysosomes. The liver 

 nuclei of animals fed ^-dimethylaminoazobenzene markedly mcrease their DNAase 

 activity (Lamirande et al., 1954). These workers have suggested a possible correlation of 

 such activity with the mitotic activity of the nuclei. 



" It has been shown that there are at least two fractions of RNA of differing meta- 

 bolic activity in nuclei (Allfrey and Mirsky, 1957; Vincent, 1957). 



