Pathway of activation by magnesium ions of substrates for the catalytic subunit of RNase P from Escherichia coli

The pathway is described for activation by Mg²⁺ of substrates for M1 RNA, the catalytic subunit of the RNase P from Escherichia coli. The dissociation constants are reported for binding of Mg²⁺ to the substrate and for the binding of the metal ion-substrate complex to the enzyme. The enzyme binds the substrate with the same affinity whether or not Mg²⁺ is already bound to the substate. However, only substrates with bound Mg²⁺ can make a productive ternary complex when combined with the enzyme. The presence of certain 2'-hydroxyl groups in the substrate is required to stabilize the binding of Mg²⁺ and, thereby, to increase the lifetime of the ternary complex. An energy profile for the reaction of M1 RNA with a small model substrate is presented and the role of Mg²⁺ bound to the substrate is discussed.