The role of the polypyrimidine stretch at the SV40 early pre‐mRNA 3′ splice site in alternative splicing.

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Abstract
We have studied the role in pre‐mRNA splicing of the nucleotide sequence preceding the SV40 early region 3′ splice site. Somewhat surprisingly, neither the pyrimidine at the highly conserved −3 position, nor the polypyrimidine stretch that extends from −5 to −15, relative to the 3′ splice site, were found to be required for efficient splicing. Mutations that delete this region or create polypurine insertions at position −2 had no significant effects on the efficiency of SV40 early pre‐mRNA splicing in vivo or in vitro. Interestingly, however, the pyrimidine content of this region had substantial effects on the alternative splicing pattern of this pre‐mRNA in vivo. Mutations that increased the number of pyrimidine residues resulted in more efficient utilization of the large T antigen mRNA 5′ splice site relative to the small t 5′ splice site, while mutations that increased the purine content enhanced small t mRNA splicing. A possible molecular mechanism for these findings, as well as a model that proposes a role for the polypyrimidine stretch in alternative splicing, are discussed.

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