Phosphoribosyl amine (PRA) is the first biosynthetic intermediate in the synthesis of purines and the hydroxymethyl moiety of thiamine pyrophosphate. In the absence of the PurF enzyme, sufficient quantities of PRA to satisfy the thiamine requirement can be generated by alternative mechanisms.


PurF-independent PRA formation involves the tryptophan operon in two different ways. Strains carrying null mutations in purF and yjgF are able to generate PRA by an unknown mechanism that requires the TrpDE complex. The TrpD and TrpE proteins form the AS-PRT (anthranilate synthase-phosphoribosyltransferase) complex and catalyze the first two steps in the biosynthesis of tryptophan.


A distinct method of PurF-independent PRA synthesis occurs in purF mutants carrying a specific class of mutations in trpC. These mutations, which do not cause auxotrophy, alter flux through the tryptophan pathway. As a result, an unstable intermediate accumulates leading to the non-enzymatic generation of PRA.


PRA generation by these tryptophan related methods are distinguishable by distinct phenotypes observed in response to exogenous tryptophan, allosteric changes to the TrpE protein, and addition of the stable tryptophan intermediate anthranilate. Anthranilate eliminates the PRA formation mediated by the lack of yjgF, but is not detrimental to flux-mediated input to PRA synthesis.


Studies are underway to determine how anthranilate inhibits PRA synthesis in a yjgF background in order to dissect the two distinct mechanisms for PRA synthesis, both of which depend on TrpDE. Understanding the integration of tryptophan and thiamine biosynthesis will provide insight into mechanisms used to create metabolic robustness within a bacterial cell.