In bacteria the biosynthetic pathways for purine mononucleotides and the hydroxymethyl pyrimidine moiety of thiamine share five reactions that result in the formation of aminoimidazole ribotide, the last metabolite common to both pathways. Here we describe the characterization of a Salmonella enterica mutant strain that has gained the ability to efficiently use exogenous aminoimidazole riboside (AIRs) as a source of thiamine. The lesion responsible for this phenotype is a null mutation in a transcriptional regulator of the GntR family (encoded by stm4068). Lack of this protein derepressed transcription of an associated operon (stm4065-4067) that encoded a predicted kinase. The stm4066 gene product was purified and shown to have AIRs kinase activity in vitro. This activity was consistent with the model presented to explain the phenotype caused by the original mutation. This mutation provides a genetic means to isolate the synthesis of the hydroxymethyl pyrimidine moiety of thiamine from the pathway for purine mononucleotide biosynthesis and thus facilitate in vivo analyses.