TPP riboswitch-dependent regulation of an ancient thiamin transporter in Candida

Riboswitches are non-coding RNA molecules that regulate gene expression by binding to specific ligands. They are primarily found in bacteria. However, one riboswitch type, the thiamin pyrophosphate (TPP) riboswitch, has also been described in some plants, marine protists and fungi. We find that riboswitches are widespread in the budding yeasts (Saccharomycotina), and they are most common in homologs of DUR31, originally described as a spermidine transporter. We show that DUR31 (an ortholog of N. crassa gene NCU01977) encodes a thiamin transporter in Candida species. Using an RFP/riboswitch expression system, we show that the functional elements of the riboswitch are contained within the native intron of DUR31 from Candida parapsilosis, and that the riboswitch regulates splicing in a thiamin-dependent manner when RFP is constitutively expressed. The DUR31 gene has been lost from Saccharomyces, and may have been displaced by an alternative thiamin transporter. TPP riboswitches are also present in other putative transporters in yeasts and filamentous fungi. However, they are rare in thiamin biosynthesis genes THI4 and THI5 in the Saccharomycotina, and have been lost from all genes in the sequenced species in the family Saccharomycetaceae, including S. cerevisiae.