EVIDENCE FOR A NOVEL GLYCINAMIDE RIBONUCLEOTIDE TRANSFORMYLASE IN ESCHERICHIA-COLI

We demonstrate here that Escherichia coli synthesizes two different gl ycinamide ribonucleotide (GAR) transformylases, both catalyzing the th ird step in the purine biosynthetic pathway. One is coded for by the p reviously described purN gene (GAR transformylase N), and a second, hi therto unknown, enzyme is encoded by the purT gene (GAR transformylase T). Mutants defective in the synthesis of the purN- and the purT-enco ded enzymes were isolated. Only strains defective in both genes requir e an exogenous purine source for growth. Our results suggest that both enzymes may function to ensure normal purine biosynthesis. Determinat ion of GAR transformylase T activity in vitro required formate as the C1 donor. Growth of purN mutants was inhibited by glycine. Under these conditions GAR accumulated. Addition of purine compounds or formate p revented growth inhibition. The regulation of the level of GAR transfo rmylase T is controlled by the PurR protein and hypoxanthine.