Metal-ion coordination drives active-site remodeling for backsplicing in circularly permuted group II introns.

Productive backsplicing in the circularly permuted group II intron Gluconacetobacter diazotrophicus PAL5 (Gdi-1) depends on ion-specific coordination networks that remodel the active site.

• Why it matters: The mechanistic rules for large-scale active-site remodeling and stringent metal-ion specificity during backsplicing are poorly understood despite structural knowledge of branching.
• What they did: High-resolution cryo-EM captured discrete stages along the branching-to-backsplicing pathway under K+/Mg2+, K+/Ca2+, and Cs+/Mg2+ conditions, enabling comparative analyses of 3' splice site docking and catalytic center formation.
• The result: A selective monovalent cation-binding site, K3, couples the γ–γ' interaction to reassembly of the heteronuclear four-metal core, promoting productive second-step active-site formation.

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