Unusually High‐Affinity Mg 2+ Binding at the AU‐Rich Sequence within the Antiterminator Hairpin of a Mg 2+ Riboswitch

Mg 2+ ‐Responsive riboswitches represent a fascinating example of bifunctional RNAs that sense Mg 2+ ions with high selectivity and autonomously regulate the expression of Mg 2+ ‐transporter proteins. The mechanism of the mgtA riboswitch is scarcely understood, and a detailed structural analysis is called for to study how this RNA can selectively recognize Mg 2+ and respond by switching between two alternative stem loop structures. In this work, we investigated the structure and Mg 2+ ‐binding properties of the lower part of the antiterminator loop C from the mgtA riboswitch of Yersinia enterocolitica by solution NMR and report a discrete Mg 2+ ‐binding site embedded in the AU‐rich sequence. At the position of Mg 2+ binding, the helical axis exhibits a distinct kink accompanied by a widening of the major groove, which accommodates the Mg 2+ ‐binding pocket. An unusually large overlap between two adenine residues on the opposite strands suggests that the bending may be sequence‐induced by strong stacking interactions, enabling Mg 2+ to bind at this so‐far not described metal‐ion binding site.