A mutational analysis of the polypyrimidine tract of introns. Effects of sequence differences in pyrimidine tracts on splicing.

The polypyrimidine (py) tract of introns is required for efficient spliceosome assembly and splicing of pre-mRNAs. A detailed mutational analysis of the py tract of an adenovirus 2 intron was carried out. Utilizing a "precursor in pieces" vector system, it was possible to synthesize py tract mutant pre-mRNAs that were otherwise identical. The mutant pre-mRNAs that were otherwise identical. The mutant pre-mRNAs were analyzed for in vitro splicing, for formation of splicing complexes, and for binding to proteins in the HeLa nuclear extract. Chimeric pre-mRNAs that contained the yeast branch point consensus sequence (UAC-UAAC) and altered py tracts were also analyzed. Mutational analysis showed the following. First, any mutation in the py tract that affected splicing did so by interferring with complex A formation in spliceosome assembly. Second, introduction of purines into the py tract is detrimental only if the length of the tract is shortened and if there is a reduction in the number of consecutive uracil residues. Third, uracil and cytosine do not have equivalent functions in the py tract. Our results with chimeric pre-mRNAs also show that a strong py tract can partially replace a weak branch point sequence and a strong branch point sequence can partially replace a weak py tract. Finally, the one surprising finding obtained when examining protein binding was that a mutant pre-mRNA did not bind to heterogeneous nuclear ribonucleoprotein C proteins and yet spliced close to wild type level.