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Cell. 1996 Mar 8;84(5):801-11.

A novel spliceosome containing U11, U12, and U5 snRNPs excises a minor class (AT-AC) intron in vitro.

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Department of Molecular Biophysics and Biochemistry, Howard Hughes Medical Institute, Yale University School of Medicine, New Haven, Connecticut 06536, USA.


A minor class of introns with noncanonical splice (AT-AC) and branch site sequences exists in metazoan protein coding genes. We have established a HeLa cell in vitro system that accurately splices a pre-mRNA substrate containing such an intron from the human P120 gene. Splicing occurs via a lariat intermediate whose branch site A residue is predicted to bulge from a duplex formed with the low abundance U12 small nuclear ribonucleoprotein (snRNP), which we confirm by psoralen cross-linking. Native gel electrophoresis reveals that U11, U12, and U5 snRNPs assemble onto the P120 pre-mRNA to form splicing complexes. Inhibition of P120 splicing by 2'-O-methyl oligonucleotides complementary to U12 or U5 demonstrates that U12 and U5 snRNPs perform essential roles in the AT-AC spliceosome.

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