A Note on the Evolution of Nucleic Acids 359 



were restored in DNAase-treated nuclei by polyadenylic acid. (We are greatly 

 indebted to Dr Severo Ochoa for his generosity in supplying this interesting 

 material.) 



Although the spectrum of suitable DNA substitutes is quite broad, it does 

 not include a number of other related, and perhaps equally Hkely, compounds. 

 For example, amino acid uptake cannot be restored by the free purine and 

 pyrimidine bases or by mixtures of nucleosides. And, although ribonucleic acid 

 will substitute for DNA, an alkaline digest of ribonucleic acid (RNA) will not. 

 By the same token, a mixture of the nucleoside 2'- and 3'- phosphates has no 

 effect on amino acid incorporation. Mixtures of the ribonucleoside 5 '-phos- 

 phates, adenosine monophosphate, adenosine diphosphate and a number of 

 dinucleotides were also tested and found inactive. (We are grateful to Dr R. B. 

 Merrifield, of the Rockefeller Institute, for the gift of these dinucleotides.) 

 Among the dinucleotides tested was adenylic-adenyHc dinucleotide, chromato- 

 graphically purified after rapid acid hydrolysis of yeast RNA; this failed to 

 promote alanine uptake in DNAase-treated nuclei. This observation takes on 

 added significance when it is compared with the previous finding that poly- 

 adenylic acid is a very effective agent in restoring alanine and leucine uptakes. 

 Thus the polynucleotide is effective where the corresponding dinucleotide is 

 not, and the molecular size of the polynucleotide emerges as one of the factors 

 which determine its capacity to promote amino acid incorporation. A clue to 

 the range of effective molecular size can be found in experiments which show 

 that the diffusible spUt-products obtained from DNA (by DNAase digestion) 

 can substitute for the DNA itself. This suggests that tri- and tetra-nucleotides 

 may be effective in this system. 



Let us now turn to another, but related, set of observations [2]. The nucleus 

 possesses the same series of mononucleotides as is foimd in the cytoplasm. These 

 nucleotides can be phosphorylated to the triphosphate from within the nucleus. 

 Phosphorylation is aerobic and is inhibited by cyanide, dinitrophenol, azide and 

 antimycin. Unlike phosphorylation in mitochondria the nuclear process is not 

 affected by carbon monoxide, calcium ions, Janus green, methylene blue or 

 dicoumarol. Incorporation of amino acids in the nucleus is closely Hnked to ATP 

 synthesis; without ATP no protein synthesis occurs. It should be noted that 

 only ATP already within the nucleus is effective; added ATP has no effect on 

 the nucleus. 



Recent experiments relate the DNA of the nucleus directly to the synthesis 

 of adenosine triphosphate [3]. This is shown by two experiments: (i) nuclei 

 pretreated with desoxyribonuclease lose their capacity to synthesize ATP; (2) 

 the capacity of DNAase-treated nuclei to synthesize ATP is readily restored 

 when they receive a DNA supplement. These experiments show that one func- 

 tion of DNA in the chromosome is to act as a 'cofactor' for ATP formation. 

 Further experiments show that ATP synthesis in DNAase-treated nuclei can be 

 restored by polynucleotides other than DNA; indeed some polynucleotides are 

 in this respect more efficient than DNA. The ability of polynucleotides to mediate 

 the synthesis of ATP would locahze the ATP at the site of protein synthesis, 

 where it is needed for amino acid activation. 



