354 GEORGE BOSWORTH BROWN AND PAUL M. ROLL 



of the purine. In L. casei a "sparing" by purines of the synthesis de novo^^ is depend- 

 ent^" on the concentrations of both the purine and the folic acid in the medium. Thus 

 the composition of the media may influence the incorporations (and the conversions) 

 observed. The comparisons referred to here are mostly based upon the addition of 

 equimolar quantities of different purines, or mixtures of purines, to a given medium 

 for each organism. 



At one extreme Torulopsis utilis^^-''^ and Aerohacter aerogenes'^ readily 

 incorporate either adenine or guanine. They extensively convert adenine 

 into polynucleotide guanine, but they convert guanine to only a very small 

 extent. Other species range, with a proportionately greater conversion of 

 guanine into polynucleotide adenine, through two strains of Paramecium 

 aurelia,^^-'^'^ Ochromonas malhamensis,''^ and E. coli,^'^'''^ to L. casei,*"'^'^ 

 which presents the maximum conversions of each purine into the poly- 

 nucleotide derivative of the other. At the other extreme Tetrahymena 

 geleii^^-^^ and L. leichmanii^* present the opposite picture. Each of these 

 organisms readily incorporates either purine and converts guanine into 

 polynucleotide adenine, but adenine was not found to be converted into 

 polynucleotide guanine by either of these species. 



The extent to which the interconversions are influenced by the presence 

 of other purines also varies. In T. utilis^^-''^ the amount of guanine derived 

 from adenine is greatly reduced when guanine is present, while in L. 

 leichmanii^^ the amount of adenine derived from guanine is reduced by the 

 presence of adenine. In L. casei^°'^" when both purines are present the in- 

 terconversion continues unabated and nearly half of each is still converted 

 into the polynucleotide derivative of the other. 



In the eight species where 2 , 6-diaminopurine has been tested,''"'^^'^^'''^' 

 77,78,81 [^ jg transformed into polynucleotide guanine, sometimes more'*^ 

 and sometimes less^^ extensively than is guanine. It is also transformed into 



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