HUMAN U2 SNRNA CAN FUNCTION IN PRE-MESSENGER-RNA SPLICING IN YEAST

THE removal of introns from messenger RNA precursors requires five small nuclear RNAs (snRNAs), contained within ribonucleoprotein particles (snRNPs), which complex with the pre-mRNA and other associated factors to form the spliceosome (reviewed in refs 1-3). In both yeast and mammals, the U2 snRNA base pairs with sequences surrounding the site of lariat formation4-9. Binding of U2 snRNP to the highly degenerate branchpoint sequence in mammalian introns is absolutely dependent on an auxiliary protein, U2AF, which recognizes a polypyrimidine stretch adjacent to the 3′ splice site10. The absence of this sequence motif in yeast introns has strengthened arguments that the two systems are fundamentally different10,11. Deletion analyses of the yeast U2 gene have confirmed that the highly conserved 5′ domain is essential, although the adjacent ̃950 nucleotides can be deleted without any phenotypic consequence12,13. A 3′-terminal domain of ̃100 nucleotides is also required for wild-type growth rates; the highly conserved terminal loop within this domain (loop IV) may provide specific binding contacts for two U2-specific snRNP proteins13-15. We have replaced the single copy yeast U2(yU2) gene with human U2(hU2), expecting that weak or no complementation would provide an assay for cloning additional splicing factors, such as U2AF. We report here that hU2 can complement the yeast deletion with surprising efficiency. The interactions governing spliceosome assembly and intron recognition are thus more conserved than previously suspected. Paradoxically, the conserved loop IV sequence is dispensable in yeast. © 1990 Nature Publishing Group.