192 



SUBCELLULAR PARTICLES 



c 

 o 



ID 



d 



4-> 



O- 



o 



o 



o 



E 

 o 



o 



c 



(1) 



CD 



c 



d 



u 



+ 40 



+ 20 



-40 



•60 



■80 



■100 



y+ ^Alanine -l-C^'* 



DNA pemoved. 

 ( pep cent of total) 



Fig. 4. Effect of removing DNA 

 from thymus nuclei upon the sub- 

 sequent incorporation of alanine- 

 i-C" and iysine-2-C". Nuclei were 

 pretreated with DNAase to remove 

 increasing amounts of their DNA. 

 They were subsequently incubated 

 for 60 minutes in the presence of 

 the isotopic amino acid. The de- 

 crease in specific activity of the 

 nuclear protein in treated nuclei, 

 relative to that observed in un- 

 treated nuclei, is plotted against the 

 percent removal of the DNA. 



20 



40 



50 



80 



100 



specificity which they demonstrate. It is not necessary to add the homologous 

 DNA. Many DNA preparations from widely diverse sources promote amino acid 

 uptake just as well as the DNA of calf thymus (table 2). Furthermore, the DNA 

 molecule need not be intact, for alkali-denatured DNA and the larger split 

 products obtained by DNAase digestion can be just as effective as the original 

 DNA preparation. Even ribonucleic acids will substitute for the DNA of the 

 thymus in restoring amino acid incorporation into nuclear proteins (table 3). The 

 lack of a specificity requirement is shown quite conclusively by experiments in 

 which both C^^-alanine and C-^^-leucine uptakes were restored in DNAase-treated 

 nuclei by the addition of polyadenylic acid. (Other experiments, which are re- 

 ferred to in the discussion of this paper, concern the restoration of synthetic func- 

 tion by other polyelectrolytes, including non-nucleotide structures.) 



Although the spectrum of suitable DNA substitutes is very broad, it does not 

 include a number of other related, and perhaps equally likely compounds. For 

 example, amino acid uptake cannot be restored to DNAase-treated nuclei by the 

 free purine and pyrimidine bases, or by mixtures of nucleosides. It was also ob- 

 served that although ribonucleic acid will substitute for DNA, an alkaline digest 

 of RNA (to yield the component nucleotides) has no effect. By the same token, 

 a mixture of the nucleoside 2'- and 3'-phosphates does not restore the capacity for 

 amino acid incorporation (table 4). Mixtures of the ribonucleoside-5'-phosphates, 

 AMP, ADP, ATP, and a number of dinucleotides were also tested and all were 

 inactive. (We are grateful to Dr. R. B. Merrifield of the Rockefeller Institute for 



