Transitions between the steps of group IIC intron splicing and mobility. (A) Forward splicing. Pre-first step, hydrolytic reactions are promoted by sequestration of DVI by π–π′. RNP assembly with the IEP dissociates π–π′, allowing DVI coordinating through IEP interactions (IIB intron: PDB 6MEC). Transition between the steps of forward splicing reforms π–π′ (PDBs 4R0D, 6ME0, 5G2X). This removes the branchpoint from the active site (gray box) allowing the second step to proceed. (B) Reverse splicing during mobility. The first mobility step accumulates intron lariat-DNA 3′ exon product. Stimulation of cDNA synthesis by dNTP addition promotes the second step of reverse splicing through IEP conformational changes. Predicted DVI dynamics are panel A in reverse.
