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.