BIOSYNTHESIS OF NUCLEIC ACIDS 391 



studies of virus reproduction carries the implication that nucleotides or 

 small polynucleotides may be transferred in the assembly of the newly 

 formed DNA. Thus the indications from the several systems studied point 

 to an importance of nucleotides in the final steps of the biosynthesis of the 

 nucleic acids, but the details of the roles of the individual nucleotides and 

 of the character of the "active" precursors must await further experimen- 

 tation. It has recently been observed^^* that an enzyme-bound AMP is in- 

 volved in the activation of acetate by coenzyme A, and suggested that such 

 an enzyme-mononucleotide complex may be of more general biosynthetic 

 import. 



In this connection some results on the growth response of L. helveticus 

 355 may be pertinent. That organism responds only to uracil, to a partial 

 hydrolysate of PNA,^^^ and to a number of dinucleotides which possess a 

 terminal cytidine-5'-phosphate.^^* The requirement does not appear to be 

 for the dinucleotides as such, but the dinucleotides have in common the 

 availabity of the specific terminal group. Conceivably "active" precursors 

 could be derivatives of this type which merely serve as carriers for the 

 transfer of a specific group. 



The question^' •^^^•^^'' as to whether the biosynthesis of polynucleotides 

 proceeds by a stepwise buildup, unit by unit, or whether there is a simul- 

 taneous assembling of all of the components is a repetition of the corre- 

 sponding debate with regard to protein synthesis. The chemical (Chapter 

 11) heterogeneity of the PNA preparations yet studied, the inequahties of 

 the incorporations of pyrimidine or purine precursors into different units 

 in a given nucleic acid, and the metaboUc^^ and chemicaP^'-^'--^ heter- 

 ogeneity (Chapter 10) of DNA preparations are all compatible with a step- 

 wise synthesis or a partial renewal. Adequate evidence regarding a simul- 

 taneous assembly will have to await availabiUty of characterized 

 homogeneous preparations of polynucleotides. 



VI. Addendum 



New information on the character of the soluble nucleotides present in 

 tissues is rapidly accumulating. Uridine-5'-triphosphate has been prepared 

 from yeast.--^ A uridine-5'-monophosphate, among others, has been recog- 

 nized in a TCA extract of hver.^-^ The presence of 5'-monophosphates, -di- 

 phosphates, and -triphosphates of uracil, cytosine, adenine, and guanine in 



224 Mary E. Jones, F. Lipmann, H. Hilz, and F. Lynen, J. Am. Chem. Soc. 75, 3286 

 (1953). 



225 E. Chargaff, C. F. Crampton, and R. Lipshitz, Nature 172, 289 (1953). 



226 G. L. Brown and M. Watson, Nature 172, 339 (1953). 



22' S. H. Lipton, S. A. Morell, A. Frieden, and R. M. Bock, J. Am. Chem. Soc. 75, 5449 



(1953). 

 228 J. Sacks, L. Lutwak, and P. D. Hurley, /. Am. Chem. Soc. 76, 424 (1954). 



