An RNA switch at the 5 ' splice site requires ATP and the DEAD box proteinPrp28p

Pre-mRNA splicing requires dramatic RNA rearrangements hypothesized to be c atalyzed by ATP-dependent RNA unwindases of the DExD/H box family. In a rea rrangement critical for the fidelity of 5' splice site recognition, a base- pairing interaction between the 5' splice site and U1 snRNA must be switche d for a mutually exclusive interaction between the 5' splice site and U6 sn RNA. By lengthening the U1:5' splice site duplex, we impeded this switch in a temperature-dependent manner and prevented formation of the spliceosome' s catalytic core. Using genetics, we identified the DExD/H box protein Prp2 8p as a potential mediator of the switch. In vitro, the switch requires bot h Prp28p and ATP. We propose that Prp28p directs isomerization of RNA at th e 5' splice site and promotes fidelity in splicing.