Unusually high-affinity Mg2+ binding at the AU-rich sequence within the antiterminator hairpin of a Mg2+ riboswitch

Abstract

Mg2+-Responsive riboswitches represent a fascinating example of bifunctional RNAs that sense Mg2+ ions with high selectivity and autonomously regulate the expression of Mg2+-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 Mg2+ and respond by switching between two alternative stem loop structures. In this work, we investigated the structure and Mg2+-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 Mg2+-binding site embedded in the AU-rich sequence. At the position of Mg2+ binding, the helical axis exhibits a distinct kink accompanied by a widening of the major groove, which accommodates the Mg2+-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 Mg2+ to bind at this so-far not described metal-ion binding site.