3' SPLICE SITE RECOGNITION IN SACCHAROMYCES-CEREVISIAE DOES NOT REQUIRE BASE-PAIRING WITH U1 SNRNA

The conserved nucleotides 9 and 10 of U1 small nuclear RNA (snRNA) hav e been proposed to base pair with either 5' exon or 3' splice site seq uences. In S. pombe, U1 snRNA pairing with the conserved 3' splice sit e is required for the first step of splicing and viability. In contras t, we show that S. cerevisiae Ul mutants at positions 9 and 10 are ful ly functional. Splicing of several genes is normal in these strains, r uling out an essential base pairing between U1 snRNA and 3' splice sit es. U1 snRNA positions 9 and 10 are shown to be involved in 5' splice site selection through their interaction with exon sequences. Our resu lts demonstrate that some snRNA-pre-mRNA interactions are not evolutio narily conserved and that 3' splice site recognition occurs by differe nt mechanisms in various organisms.