THE ROLE OF PURINE BASES AS HISTIDINE PRECURSORS IN Lactobacillus Casei.

The requirement of Lactobacillus casei and Lactobacillus arabinosus for purine bases is increased when these organisms are grown in the absence of histidine. These and related observations were interpreted to mean that purine bases serve as precursors of histidine for these organisms.1 Recent studies by Levy and Coon2'3 have established that in yeast the imidazole ring of purine bases does not appear in histidine and that formate is an excellent source of the amidine carbon (position 2) of the imidazole ring of histidine. They suggested that purine bases might serve primarily as donors of formate in the synthesis of histidine by lactic acid bacteria. We have now compared the incorporation of variously labeled guanine-C'4 and of formate-C"4 into histidine under various conditions. The results obtained show that in lactic acid bacteria, as in yeast, the imidazole ring of purine bases does not serve as precursor for that of histidine. However, carbon 2 of guanine serves efficiently as a precursor of position 2 of the imidazole ring of histidine and enters this position readily under conditions of folic acid deficiency that prevent incorporation of formate-C 4.