Kinetics of an RNA conformational switch.

The spliced leader RNA from Leptomonas collosoma has two competing secondary structures of nearly equal free energy. Short, complementary oligonucleotides can drive the structure from one form of the other. We report stopped-flow rapid-mixing and temperature-jump measurements of the kinetics of the structural switch. At high concentrations of oligonucleotide, the rate of binding becomes limited by the rate of the structural switch, which occurs on a time scale of a fraction of a second. The low activation energy observed for the process implies a branch migration type of mechanism in which portions of the two competing helices transiently coexist.