Ligand binding and gene control characteristics of tandem riboswitches in Bacillus anthracis

Most riboswitches are composed of a single metabolite-binding aptamer and a single expression platform that function together to regulate genes in response to changing metabolite concentrations. In rare instances, two aptamers or sometimes two complete riboswitches reside adjacent to each other in untranslated regions (UTRs) of mRNAs. We have examined an example of a tandem riboswitch in the Gram-positive bacterium Bacillus anthracis that includes two complete riboswitches for thiamine pyrophosphate (TPP). Unlike other complex riboswitch systems described recently, tandem TPP riboswitches do not exhibit cooperative ligand binding and do not detect two different types of metabolites. In contrast, both riboswitches respond independently to TPP and are predicted to function in concert to mimic the more "digital" gene control outcome observed when two aptamers bind ligands cooperatively. Our findings further demonstrate that simple gene control elements made only of RNA can be assembled in different architectures to yield more complex gene control outcomes.