BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 513 



If phosphorylated forms of PNA really exist, they might raise the energy 

 level of activated amino acids on the surface of the cytoplasmic template 

 (microsomes). But other possibilities for energy-rich bonds within the 

 microsomes still exist and should not be forgotten. It is known, for instance, 

 as a result of Brachet and Jeener's'^^ early work, that cytoplasmic particles 

 give strong reactions for — SH groups; and it is now a well-established 

 fact that — S ~ PO3H2 bonds are energy rich (Lipmann.^i^ Walsh^^o). The 

 possible significance of such bonds is strengthened by Peterson and Green- 

 berg's-'^' observation that a considerable proportion of the radioactivity of 

 mitochondria incubated with labeled amino acids is removed by interaction 

 with mercaptoethanol. Such — S '^ PO3H2 groups might represent a 

 counterpart in the microsomes of the phosphoarginine groups which have 

 been postulated by Stern--- to account for gene synthesis in chromosomes. 

 Still another possibility lies in the fact, recently discovered by Wieland 

 and Schneider--^ and by Stadtman and White,-* that acetyl derivatives of 

 imidazole are energy-rich compounds: these findings suggest that the 

 imidazole ring of histidine might take part in the transfer or activation of 

 acyl groups, but similar conditions might also prevail in the case of the 

 purines. 



In summary, it may be said that the work of recent years has considera- 

 bly strengthened the hypothesis that PXA plays a role in protein synthesis; 

 the evidence comes from cytochemical observations, studies on plant 

 virus multiplication, analysis of morphogenesis, biochemical experiments 

 on the uptake of labeled amino acids, etc. But, however convincing, this 

 evidence still remains circumstantial : what is required now is the develop- 

 ment of new methods for the isolation of "native" PNA's and decisive 

 experiments for testing the biochemical and biological properties of these 

 substances. Such experiments might very well show that PNA, like DNA 

 in the phage and transforming principle systems, plays an important 

 genetic role and that it acts as a catalyst in protein synthesis. 



IV. Addendum 



So many papers have been devoted, during the past few months, to the 

 various aspects of the biological role of PXA that the most important 

 only can be briefly summarized here. 



In the field of plant viruses, valuable contributions have been made by 



219 F. Lipmann, Bacterial. Revs. 17, 1 (1953). 



220 E. O. F. Walsh, Nature 169, 546 (1952). 



2" E. A. Peterson and D. M. Greenberg, J. Biol. Chem. 194, 359 (1952). 



"2 K. G. Stern, Yale J. Biol, and Med. 19, 937 (1947). 



"3 T. Wieland and G. Schneider, Ann. 580, 159 (1953). 



2" E. R. Stadtman and F. H. White, /. Am. Chem. Soc. 75, 2022 (1953). 



