Influence of the A15 mutation on the conformational energy balance in Escherichia coli tRNA^Tyr

The A15 mutation in Escherichia coli tRNA_su3^Tyr produces a transfer RNA whose tertiary structure has either a higher or lower t_m than the wild type, depending on Mg²⁺ concentration. The enthalpy that stabilizes the tertiary structure is greatly reduced by the A15 mutation, but there are large compensating entropy changes. At 37 °C the mutation decreases the magnitude of the free energy stabilizing the tertiary structure for all Mg²⁺ concentrations. The nucleotide modifications s⁴U, iA and G^* do not contribute detectably to tertiary structure stability. The results can be interpreted in terms of a tertiary pairing between A15 and C57 in tRNA_{su3 A15}^Tyr, of a form suggested by the unusual bonding between G15 and C48 found in crystallographic studies of yeast tRNA^phe. The observed disturbance in the conformational energy balance should contribute to the defective function of tRNA_{su3 A15}^Tyr.