GENETIC-EVIDENCE FOR BASE-PAIRING BETWEEN U2 AND U6 SNRNA IN MAMMALIAN MESSENGER-RNA SPLICING

REMOVAL of introns from eukaryotic nuclear messenger RNA precursors is catalysed by a large ribonucleoprotein complex called the spliceosome, which consists of four small nuclear ribonucleoprotein particles (U1, U2, U5, and U4/U6 snRNPs) and auxiliary protein factors 1,2. We have begun a genetic analysis of mammalian U2 snRNA by making second-site mutations in a suppressor U2 snRNA 3. Here we find that several mutations in the 5' end of U2 (nucleotides 3-8) are deleterious and that one of these can be rescued by compensatory base changes in the 3' end of U6 (nucleotides 92-95). The results demonstrate genetically that the base-pairing interaction between U2 (nucleotides 3-11) and U6 snRNA (nucleotides 87-95), originally proposed on the basis of psoralen photocrosslinking experiments 4, can influence the efficiency of mRNA splicing in mammals. The U2/U6 interaction in yeast, however, is fairly tolerant to mutation 5 (D. J. Field and J. D. Friesen, personal communication), emphasizing the potential for facultative RNA interactions within the spliceosome.