386 GEORGE BOSWORTH BROWN AND PAUL M. ROLL 



is not, and that the interconversions of the ribosyl purines are different 

 from the corresponding free bases, suggest that utihzation of nucleosides 

 is not via the free bases. Also, since the nucleosides are utilized less effec- 

 tively than the free bases (which is also true for xanthosine^-), the free 

 bases are probably not first converted to the ribosides. 



In the L. casei there was an extreme difference between the utihzation of 

 the purine moieties of the isomeric purine nucleotides. The incorporations^^ 

 of the 3'-isomers were essentially identical with those of the corresponding 

 free purines and were far greater than those of the nucleosides. In contrast, 

 there was no significant utilization of the 2'-isomers. The 3'-isomers of the 

 nucleotide of either purine also support growth (in folic acid-free medium) 

 as effectively as do the purines, while the 2'-isomers and adenosine-5'-phos- 

 phate are very much less effective. ^^ A specific response to the 3'-isomers of 

 nucleotides is also shown by several purine-reciuiring mutants of E. coli 

 and Bacillus suhiilis. 



The facts that adenosine-3'-phosphate is not effective in reversing the 

 inhibitory effects of diaminopurine^^ although adenine does effect that 

 reversal, and that adenosine-3'-phosphate is efficiently utilized by a di- 

 aminopurine-resistant mutant which does not utilize adenine readily,^' 

 also suggest that the nucleotide is not metabolized via the free purine. The 

 extent of utilization of the 3'-nucleotides is compatible with an assumption 

 that the 3'-nucleotides could be one of the first products formed from the 

 free purines in this species. However, the contrast with the response of the 

 rat, which utilizes the 2'- and 3'-isomers of adenylic acid to equal extents,^^ 

 makes any generalization impossible. The poor utilization of adenosine-5'- 

 phosphate for growth suggests that that compound may not be the "active" 

 polynucleotide precursor. 



The pertinence of the presence in L. casei of a specific 3'-nucleotidase liberating 

 inorganic phosphate'^^ is not clear since there is no evidence of a degradation of the 

 nucleotides in the course of their incorporation. The incorporation by L. leichmanii^^ 

 of the ribose as well as the purine of a guanylic acid (isomerism not specified) is in 

 accord with the evidence for direct utilization of nucleotides by L. casei -j^^ however, 

 the cleavage of cytidine during its utilization by E. coli, but not by the rat,*^ again 

 suggests restraint in interpretation. 



The question of differential permeability is occasionally raised in attempts to 

 explain seemingly anomalous utilizations, but is frequently unapproachable experi- 

 mentally. However, in the cases of the utilization of the 3'-nucleotides by L. casei, 

 of guanylic acid by L. leichmanii ,^^ and of certain dinucleotides by L. helveticus ,^^^ ■'^^^ 

 the factor of permeability would seem to be excluded. 



192 L. Shuster and N. O. Kaplan, J. Biol. Chem. 201, 535 (1953). 

 '93 R. B. Merrifield and M. S. Dunn, J. Biol. Chem. 186, 331 (1950). 

 19-' R. B. Merrifield and D. W. Woolley, J. Biol. Chem. 197, 521 (1952). 



