ENRICHMENT FOR RNA MOLECULES THAT BIND A DIELS-ALDER TRANSITION-STATEANALOG

RNA molecules that bind a transition state analog for a Diels-Alder re action (K-d = 0.35 +/- 0.05 mM) were isolated from a starting pool of approximate to 10(14) sequences by affinity chromatography. After the initial rise and plateau of the amount of RNA that eluted with soluble analog, a step gradient elution was used to further enrich the pool f or sequences with higher affinities for the target. To our knowledge, the isolation of RNA molecules that bind either a nonplanar or a hydro phobic ligand has not been reported previously. A conserved nucleotide sequence and secondary structure present in many of the RNA molecules are necessary but not sufficient for binding the analog. No catalysts of the targeted Diels-Alder reaction were found among the binders. Th e absence of catalysis contrasts with previous successful experiments with antibodies and suggests that other strategies may be needed to id entify oligonucleotides with diverse catalytic activities.