Comparative Genomic and Genetic Evidence on a Role for the OarX Protein in Thiamin Salvage

Salvage pathways for thiamin and its thiazole and pyrimidine moieties are poorly characterized compared to synthesis pathways. A candidate salvage gene is oarX, which encodes a short-chain dehydrogenase/reductase. In diverse bacteria, oarX clusters on the chromosome with genes of thiamin synthesis, salvage, or transport and is preceded by a thiamin pyrophosphate riboswitch. Thiamin and its moieties can undergo oxidations that convert a side-chain hydroxymethyl group to a carboxyl group, or the thiazole ring to a thiazolone, causing a loss of biological activity. To test if OarX participates in salvage of the carboxyl or thiazolone products, we used a genetic approach in Corynebacterium glutamicum ATCC 14067, which is auxotrophic for thiamin's pyrimidine moiety. This strain could not utilize the pyrimidine carboxyl derivative. This excluded a role in salvaging this product and narrowed the function search to metabolism of the carboxyl or thiazolone derivatives of thiamin or its thiazole moiety. However, a ΔthiG (thiazole auxotroph) strain was not rescued by any of these derivatives. Nor did deleting oarX affect rescue by the physiological pyrimidine and thiazole precursors of thiamin. These findings reinforce the genomic evidence that OarX has a function in thiamin metabolism and rule out five logical possibilities for what this function is.