MUTATIONS IN U1 SNRNA BYPASS THE REQUIREMENT FOR A CELL TYPE-SPECIFICRNA SPLICING FACTOR

Previous studies have demonstrated that efficient splicing of the prim ary transcript of the yeast MER2 gene requires the MER1 protein, which is produced only in meiotic cells. A genetic selection was devised to recover second-site mutations that bypass the requirement for MER1 in MER2 RNA splicing. This selection identified a mutation in SNR19, the gene for U1 snRNA. The suppressor mutation affects the first residue in U1 snRNA, allowing this nucleotide to base pair with the eighth nuc leotide in the MER2 intron. This base in MER2 lies outside the conserv ed hexanucleotide that defines the 5' splice site in yeast. The MER2 5 ' splice site (GUUCGU) differs from the consensus in yeast (GUAYGU) at the third position. When this nucleotide is mutated to restore the co nsensus, base pairing with U1 snRNA is increased and the requirement f or MER1 is alleviated.