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Splicing of a circular yeast pre-mRNA in vitro.

We have tested the fate of a circularized synthetic pre-mRNA transcript in a whole cell splicing extract of Saccharomyces cerevisiae. Our results demonstrate that this circular precursor RNA is able to induce spliceosome formation in vitro and that the products of the following splicing reaction are the lariat-shaped intron, and a mature circular mRNA. Thus, it would appear that free 5' and/or 3' ends are not obligatory for a splicing reaction to occur, although we find its efficiency to be strongly influenced by the presence or lack of free ends. To our knowledge, this is the first demonstration that a circular pre-mRNA molecule is recognized as a suitable substrate by an eukaryotic mRNA splicing apparatus.