Genetic blocks and unique features in the biosynthesis of 5'-phosphoribosyl-N-formylglycinamide in Salmonella typhimurium.

Urine-requiring mutants of Salmonella fyphimurium have been examined for the ability to synthesize 5’-phosphoribosyl-IV-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 I-pyrephosphate 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 formyltrausferase system different from the equivalent nonbacterial systems.