An essential yeast snRNA with a U5-like domain is required for splicing in vivo.

Yeast contains at least 24 snRNAs, many of which are dispensable for viability. We recently demonstrated that a small subset of these RNAs has a functional binding site for the Sm antigen, a hallmark of metazoan snRNAs involved in mRNA processing. Here we show that one of these snRNAs, snR7, is required for growth. To determine the biochemical basis of lethality in cells lacking snR7, we engineered the conditional synthesis of snR7 by fusing the snRNA coding sequences to the yeast GAL1 control region. Cells depleted for the SNR7 gene product by growth on glucose for five generations show marked accumulation of unspliced mRNA precursors from the four intron-containing genes tested. In some cases, intron-exon 2 lariats also accumulate. We have identified a 70 nucleotide domain within snR7 with limited sequence-specific but striking structural homology to the mammalian snRNA U5. We conclude that mRNA splicing in yeast requires the function of a U5-like snRNA.