RNA FOLDING AT MILLISECOND INTERVALS BY SYNCHROTRON HYDROXYL RADICAL FOOTPRINTING

Radiolysis of water with a synchrotron x-ray beam permits the hydroxyl radical-accessible surface of an RNA to be mapped with nucleotide res olution in 10 milliseconds. Application of this method to folding of t he Tetrahymena ribozyme revealed that the most stable domain of the te rtiary structure, P4-P6, formed cooperatively within 3 seconds. Exteri or helices became protected from hydroxyl radicals in 10 seconds, wher eas the catalytic center required minutes to be completely folded. The results show that rapid collapse to a partially disordered state is f ollowed by a slow search for the active structure.