Allosteric interpretation of Mg2+ binding to the denaturable Escherichia coli tRNAGlu2+.

The Mg2+ binding properties of the denaturable tRNAGlu2 from E. coli in 0.1 M Na+, pH7, are characterized by equilibrium dialysis. At 34 degrees C, where the native and denatured conformers are in equilibrium, Mg2+ binding is cooperative. By trapping the tRNA completely in the native conformation at 4 degrees C it is shown that native tRNAGlu2 possesses one strong binding site, K1 = 7.5 x 10(4) M-1 and approximately 36 weak sites with K2 = 8.3 x 10(2) M-1. A significantly lower affinity for the denatured conformer is indicated. We show that Mg2+ effects an allosteric transition from the low affinity denatured conformational state to the high affinity native state and develop the appropriate equations to fit the Mg2+ binding data with physically meaningful parameters. Our results also suggest the previously reported cooperative cation binding to tRNA arises from a cation induced conformational change to the native tRNA conformation and does not reflect the inherent Mg2+ binding properties of the native conformer.