Convergent Use of Heptacoordination for Cation Selectivity by RNA and Protein Metalloregulators

The large yybP-ykoY family of bacterial riboswitches is broadly distributed phylogenetically. Previously, these gene-regulatory RNAs were proposed to respond to Mn²⁺. X-ray crystallography revealed a binuclear cation-binding pocket. This comprises one hexacoordinate site, with six oxygen ligands, which preorganizes the second, with five oxygen and one nitrogen ligands. The relatively soft nitrogen ligand was proposed to confer affinity for Mn²⁺, but how this excludes other soft cations remained enigmatic. By subjecting representative yybP-ykoY riboswitches to diverse cations in vitro, we now find that these RNAs exhibit limited transition metal ion selectivity. Among the cations tested, Cd²⁺ and Mn²⁺ bind most tightly, and comparison of three new Cd²⁺-bound crystal structures suggests that these riboswitches achieve selectivity by enforcing heptacoordination (favored by high-spin Cd²⁺ and Mn²⁺, but otherwise uncommon) in the softer site. Remarkably, the Cd²⁺- and Mn²⁺-selective bacterial transcription factor MntR also uses heptacoordination within a binuclear site to achieve selectivity.