The incorporation of exogenous purines into pentose nucleic acid by Lactobacillus casei.

The complex question of the biosynthetic origin of nucleic acid purines has been approached from a number of directions. The elaboration of the over-all picture for a single organism has obvious advantages, and the experiments reported here represent a study in that direction with Lactobacillus casei as the test organism. Extensive information regarding purine utilization by L. casei is available. This microorganism requires either folic acid or thymine and a purine for growth. In the thymine-containing medium, adenine, guanine, hypoxanthine, and xanthine serve equally well as sources of purine. Since the total quantity of purine consumed during growth approximates that found in the cells produced, the nutritional requirement of the microorganism for purine is presumptive evidence for the interconversion of the physiological purines (1). The conversion of exogenous adenine-g-Cl4 into pentose nucleic acid (PNA) adenine and guanine has been shown (2). The conversion of guanine-8-Cl4 into both PNA purines and, in the presence of equimolar quantities of adenine and guanine, the continued interconversion of the two purines have also been demonstrated (3). Although 2,6-diaminopurine can be converted into PNA guanine by mammals (4, 5), marked inhibitory effects of 2,6-diaminopurine on a number of biological systems have been reported.’ Elion and Hitchings (8) studied the inhibitory effect of 2 ,Bdiaminopurine on the growth of L. casei. In a thymine-containing medium adenine was found to be more effective than other purines in the restoration of growth of an inhibited system. In the medium containing folic acid, diaminopurine was found to be a strong inhibitor. At low concentrations of the inhibitor, growth