NMR spectroscopy has revealed pH-dependent structural changes in the highly conserved catalytic domain 5 of a bacterial group II intron. Two adenines with pK(a) values close to neutral pH were identified in the catalytic triad and the bulge. Protonation of the adenine opposite to the catalytic triad is stabilized within a G(syn)-AH(+) (anti) base pair. The pH-dependent anti-to-syn flipping of this G in the catalytic triad modulates the known interaction with the linker region between domains 2 and 3 (J23) and simultaneously the binding of the catalytic Mg(2+) ion to its backbone. Hence, this here identified shifted pK(a) value controls the conformational change between the two steps of splicing.

NMR spectroscopyh as revealed pH-dependent structural changes in the highly conserved catalytic domain 5 of ab acterial group II intron. Tw oa denines with pK a values close to neutral pH were identified in the catalytic triad and the bulge.Protonation of the adenine opposite to the catalytic triad is stabilized within aG (syn)-AH + (anti)b ase pair.T he pHdependent anti-to-syn flipping of this Gi nt he catalytic triad modulates the knowni nteraction with the linker region between domains 2and 3(J23) and simultaneously the binding of the catalytic Mg 2+ ion to its backbone.H ence,t his here identified shifted pK a value controls the conformational change between the two steps of splicing. Angewandte Chemie 9825

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