Saccharomyces cerevisiae has a U1-like small nuclear RNA with unexpected properties

Previous experiments indicated that only a small subset of the approximately equal to 24 small nuclear RNAs (snRNAs) in Saccharomyces cerevisiae have binding sites for the Sm antigen, a hallmark of metazoan small nuclear ribonucleoproteins (snRNPs) involved in pre-messenger RNA splicing. Antibodies from human serum to Sm proteins were used to show that four snRNAs (snR7, snR14, snR19, and snR20) can be immunoprecipitated from yeast extracts. Three of these four, snR7, snR14, and snR20, have been shown to be analogs of mammalian U5, U4, and U2, respectively. Several regions of significant homology to U1 (164 nucleotides) have now been found in cloned and sequenced snR19 (568 nucleotides). These include ten out of ten matches to the 5' end of U1, the site known to interact with the 5' splice site of mammalian introns. Surprisingly, the precise conservation of this sequence precludes perfect complementarity between snR19 and the invariant yeast 5' junction (GTATGT), which differs from the mammalian consensus at the fourth position (GTPuAGT).