AN ENGINEERED RIBONUCLEASE PREFERRING PHOSPHOROTHIOATE RNA

We used mutants of RNase Tl and the R-P isomer of a thiosubstituted su bstrate to determine stereospecific thioeffects on catalysis. The anal ysis reveals subtle structural and functional changes in the intermole cular transition state interactions. Tyr 38 contributes to catalysis t hrough a hydrogen bond with the pro-R-P oxygen. Y38F RNase Tl prefers the R-P thiosubstituted analog over the natural phosphodiester substra te that is favored by the wild type enzyme.