An RNA mutation that increases the affinity of an RNA-protein interaction.

The introduction of a cytidine in place of one of the two single stranded uridines in the R17 replicase translational operator results in a much tighter binding to R17 coat protein. The complex containing the variant RNA is stable to gel electrophoresis and has a binding constant about 50 times greater than the one with wild type RNA. The nearly thirty percent increase in the free energy of binding for the variant RNA is primarily due to a more favorable enthalpy of interaction. A possible explanation for this surprising result is that the U to C change leads to a greater extent of formation of a transient covalent complex between the protein and the RNA.