Splicing of the Meiosis-Specific HOP2 Transcript Utilizes a Unique 5′ Splice Site

TheSaccharomyces cerevisiae HOP2 gene is required to prevent formation of synaptonemal complex between nonhomologous chromosomes during meiosis. TheHOP2 gene is expressed specifically in meiotic cells, with the transcript reaching maximum abundance early in meiotic prophase. TheHOP2 coding region is interrupted by an intron located near the 5′ end of the gene. This intron contains a nonconsensus 5′ splice site (GUUAAGU) that differs from the consensus 5′ splice signal (GUAPyGU) by the insertion of a nucleotide and by a single nucleotide substitution. Bases flanking theHOP2 5′ splice site have the potential to pair with sequences in U1 small nuclear RNA, and mutations disrupting this pairing reduce splicing efficiency.HOP2 pre-mRNA is spliced efficiently in the absence of the Mer1 and Nam8 proteins, which are required for splicing the transcripts of two other meiosis-specific genes.