REQUIREMENT OF U12 SNRNA FOR IN-VIVO SPLICING OF A MINOR CLASS OF EUKARYOTIC NUCLEAR PRE-MESSENGER-RNA INTRONS

A conserved sequence element in a minor class of eukaryotic pre-messen ger RNA (pre-mRNA) introns was previously proposed to base pair with a complementary sequence in the U12 small nuclear RNA (snRNA) in a mann er analogous to the pairing of U2 snRNA with the branch site sequence of the major class of introns. Here, mutations generated in this conse rved sequence element block the splicing of a member of this minor int ron class in vivo. This block was relieved by coexpression of a U12 sn RNA containing compensatory mutations that restore the proposed base p airing interaction, These results show that this minor class of pre-mR NA introns is a distinct class existing alongside the major class of i ntrons in animal genomes, and these results also establish an in vivo function for U12 snRNA.