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.