The purine riboswitch is a genetic regulatory element found in the
5'-untranslated regions of Gram-positive bacteria that regulates expression of
the mRNA specifically in response to either guanine or adenine. We report that
the adenine-responsive RNA element is also capable of specifically recognizing
pyrimidine compounds bearing modifications at the 6- or 5,6-positions in a
fashion similar to that of purine compounds. Using isothermal titration
calorimetry and X-ray crystallography, the binding of these compounds is
characterized.
