Mutations in the tryptophan operon allow PurF-independent thiamine synthesis by altering flux in vivo

Phosphoribosyl amine (PRA) is an intermediate in purine biosynthesis and also required for thiamine biosynthesis in Salmonella enterica. PRA is normally synthesized by phosphoribosyl pyrophosphate (PRPP) amidotransferase, a high-turnover enzyme of the purine biosynthetic pathway encoded by purF. However, PurF-independent PRA synthesis has been observed in strains with different genetic backgrounds and growing under diverse conditions. Genetic analysis has shown that the anthranilate synthase-phosphoribosyltransferase (AS-PRT) enzyme complex, involved in the synthesis of tryptophan, can play a role in the synthesis of phosphoribosyl amine (PRA). This work describes the in vitro synthesis of PRA in the presence of the purified components of AS-PRT complex. Results from in vitro assays and in vivo studies indicate the cellular accumulation of phosphoribosyl anthranilate can result in non-enzymatic PRA formation sufficient for thiamine synthesis. These studies have uncovered a mechanism used by cells to redistribute metabolites to ensure thiamine synthesis, and may define a general paradigm of metabolic robustness.