INTERACTION BETWEEN THE FIRST AND LAST NUCLEOTIDES OF PRE-MESSENGER-RNA INTRONS IS A DETERMINANT OF 3' SPLICE-SITE SELECTION IN SACCHAROMYCES-CEREVISIAE

The splicing of group II and nuclear pre-mRNAs introns occurs via a similar splicing pathway and some of the RNA-RNA interactions involved in these splicing reactions show structural similarities. Recently, genetic analyses performed in a group II intron and the yeast nuclear actin gene suggested that non Watson-Crick interactions between intron boundaries are important for the second splicing step efficiency in both classes of introns. We here show that, in the yeast nuclear rp51A intron, a G to ($) under bar A mutation at the first position activates cryptic 3' splice sites with the sequences UA ($) under bar C/ or UA ($) under bar A/. Moreover, the natural 3' splice site could be reactivated by a G to ($) under bar C substitution of the last intron nucleotide. These results demonstrate that the interaction between the first and last intron nucleotides is a conserved feature of nuclear pre-mRNA splicing in yeast and is involved in the mechanism of 3' splice site selection.