The U2 snRNP binds to the site of branch formation during splicing of mammalian pre-mRNA in vitro. In Saccharomyces cerevisiae the branch site is within the so-called TACTAAC box (UACUAAC box), an absolutely conserved intron sequence required for splicing. Based on the identification and sequence of a U2 analogue in yeast, a specific base pairing interaction between the UACUAAC box and a highly conserved region of this snRNA can be proposed. To test this hypothesis, we have taken advantage of two mutations constructed previously in the UACUAAC box of an actin-HIS4 fusion. These mutant strains were transformed with stable plasmids bearing U2-like snRNAs into which changes predicted to restore base pairing had been introduced. Aliele-specific suppression of biological and biochemical phenotypes was observed in both cases. Recognition of the UACUAAC box thus relies, at least in part, on Watson-Crick base pairing with the yeast U2 analogue.
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We are grateful to M. Ares for communicating results prior to publication. We thank L. Esperas for excellent technical assistance, and J. Pic-cini for help in preparation of the manuscript. This work was supported by grants GM21119 from the National Institutes of Health and DCB-6603926 from the National Science Foundation to C. G. R G. S. was supported by a Damon Runyon-Walter Winchell Cancer Fund Fellowship (DRG-672).