Phosphoribosylamine (PRA) is an intermediate in the biosynthetic pathway that is common to thiamine and purines. Glutamine phosphoribosyl pyrophosphate (PRPP) amidotransferase is the product of the purF gene in Salmonella enterica and catalyzes the synthesis of PRA from PRPP and glutamine. Strains lacking PurF require exogenous addition of purines for growth. However, under some growth conditions or with specific secondary mutations these strains grow in the absence of exogenous thiamine. Mutant alleles of hisA, which encodes 1-(5-phosphoribosyl)-5-[(5-phosphoribosylamino) methylideneamino] imidazole-4-carboxamide (ProFAR) isomerase, allowed PurF-independent PRA formation. The alleles of hisA that suppressed the requirement for exogenous thiamine resulted in proteins with reduced enzymatic activity. Data presented here showed that decreased activity of HisA altered metabolite pools and allowed PRA formation from ProFAR. Possible mechanisms of this conversion were proposed. The results herein emphasize the plasticity of the metabolic network and specifically highlight the potential for chemical syntheses to contribute to network robustness.