Differential role of the intermolecular base-pairs G292-C(75) and G293-C(74) in the reaction catalyzed by Escherichia coli RNase P RNA

We present a systematic investigation of the thermodynamic and kinetic role of the intermolecular G292-C(75 )and G293-C(74 )Watson-Crick base-pairs in the reaction catalyzed by Escherichia coli RNase P RNA. Single turnover kinetics were analyzed for wild-type RNase P RNA and two variants with a single G to C exchange (C292 or C293), either acting on wild-type precursor tRNA (ptRNA) or derivatives carrying a complementary change at the tRNA 3'-end (G(74)CA or CG(75)A). Ground state binding of tRNA was studied using three different methods, including a novel fluorescence-based assay measuring equilibrium binding. We conclude that: (1) the role of the G293-C(74 )interaction is essentially confined to Watson-Crick base-pairing, with no indication for crucial tertiary contacts involving this base-pair; (2) the G293-C(74 )pair, although being as important for ptRNA ground state binding as G292-C(75), is much less crucial to catalytic performance than the G292-C(75) pair; (3) disruption of the G292-C(75 )base-pair results in preferential destabilization of enzyme transition-state complexes; and (4) the identity of the G292-C(75) pair, as part of the higher-order structural context consisting of coplanar G292-C(75)-A258 and G291-G259-A(76 )triples, contributes to high affinity binding of ptRNA and catalytic efficiency.