Genetic Blocks and Unique Features in the Biosynthesis of 5'-Phosphoribosyl-N-formylglycinamide in Salmonella typhimurium

Purine-requiring mutants of Salmonella typhimurium have been examined for the ability to synthesize 5'-phosphoribosyl- N -formylglycinamide in a coupled reaction involving the first three enzymes of the biosynthetic pathway for purine nucleotides. Extracts prepared from mutants belonging to the distinct genetic classes, pur F and pur D , were inactive in this assay but were active when mixed together. Further analysis showed that pur F mutants were deficient in the first enzyme, 5-phosphoribosyl 1-pyrophosphate amidotransferase (ribosylamine-5-phosphate:pyrophosphate phosphoribosyltransferase, EC 2.4.2.14), and pur D mutants were apparently deficient in the second enzyme, phosphoribosylglycinamide synthetase (ribosylamine-5-phosphate:glycine ligase, EC 6.3.1.3). The third enzyme, phosphoribosylglycinamide formyltransferase (5'-phosphoribosyl- N -formylglycineamide:tetrahydrofolate 5,10-formyltransferase, EC 2.1.2.2), was present in all mutants, and a genetic deficiency for this enzyme has not been found. Attempts to limit the action of this enzyme by creating folate deficiencies were unsuccessful. This and other considerations suggest a uniqueness in the formyltransferase system different from the equivalent nonbacterial systems.