Specific alterations of U1-C protein or U1 small nuclear RNA can eliminatethe requirement of Prp28p, an essential DEAD box splicing factor

While some members of the ubiquitous DExD/H box family of proteins have RNA helicase activity in vitro, their roles in vivo remain virtually unknown. Here, we show that the function of an otherwise essential DEAD box protein, Prp28p, can be bypassed by mutations that alter either the protein U1-C or the U1 small nuclear RNA. Further analysis suggests that the conserved L13 residue in the U1-C protein makes specific contact to stabilize the U1 snR NA/5' splice site duplex in the prespliceosome, and that Prp28p functions t o counteract the stabilizing effect of the U1-C protein, thereby promoting the dissociation of the U1 small nuclear ribonucleoprotein particle from th e 5' splice site. Thus, in addition to unwinding RNA, the DExD/H box protei ns may affect RNA-RNA rearrangements by antagonizing specific RNA-stabilizi ng proteins.