The products of splicing of simian virus 40 early pre mRNA in HeLa cell nuclear extracts have been characterized. Of the two alternative splicing patterns exhibited by this precursor in vivo, which ...involve the use of alternative large T and small t 5′ splice sites and a single shared 3′ splice site, only one, producing large T mRNA, was found to occur in vitro. A number of possible intermediates and byproducts of splicing of large T mRNA were observed, including free large T 5′ exon, lariat form intron joined to 3′ exon and free lariat and linear forms of large T intron. The formation of these products argues strongly for a basic similarity in the mechanism underlying large T and other, non-alternative splices. A collection of RNAs resulting from protection of early pre mRNA at specific points from an endogenous 5′ to 3′ exonuclease activity in vitro have also been observed. The regions of the precursor RNA protected map to positions immediately upstream of the 5′ splice sites of large T and small t and the lariat branchpoint, and may represent interaction of these regions with components of the splicing machinery.
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