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Science 13 June 1997:
Vol. 276. no. 5319, p. 1621
DOI: 10.1126/science.276.5319.1621r
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A new approach for analyzing the sequences and factors for exon ligation, the second step in the splicing of nuclear precursors to messenger RNA (pre-mRNA), has revealed that this process in the human spliceosome does not require covalent attachment of a 3' splice site to the branch point. Anderson and Moore studied this process with a pre-mRNA that lacked the 3' splice site and had no sequences 3' to the pyrimidine-rich region that defines the branch point in mammals. Splicing intermediates accumulated that could then react with RNA containing the 3' splice site. Although this bimolecular attachment would tend to be inhibited in vivo with full-length substrates, these results show that the need for a covalently attached polypyrimidine tract or of specific 3' exon sequences is not absolute.

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