KINETICS OF AN RNA CONFORMATIONAL SWITCH

The spliced leader RNA from Leptomonas collosoma has two competing sec ondary structures of nearly equal free energy. Short, complementary ol igonucleotides can drive the structure from one form to the other. We report stopped-flow rapid-mixing and temperature-jump measurements of the kinetics of the structural switch. At high concentrations of oligo nucleotide, the rate of binding becomes limited by the rate of the str uctural 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 hel ices transiently coexist.