An exon that prevents transport of a mature mRNA

In Caenorhabditis elegans, pre-mRNA for the essential splicing factor U2AF( 65) sometimes is spliced to produce an RNA that includes an extra 216-bp in ternal exon, exon 3, Inclusion of exon 3 inserts an in-frame stop codon, ye t this RNA is not subject to SMG-mediated RNA surveillance. To test whether exon 3 causes RNA to remain nuclear and thereby escape decay, we inserted it into the 3' untranslated region of a gfp reporter gene. Although exon 3 did not affect accumulation or processing of the mRNA, it dramatically supp ressed expression of green fluorescent protein (GFP), We showed by in situ hybridization that exon 3-containing gfp RNA is retained in the nucleus, In triguingly, exon 3 contains 10 matches to the 8-bp 3' splice-site consensus . We hypothesized that U2AF might recognize this octamer and thereby preven t export. This idea is supported by RNA interference experiments in which r educed levels of U2AF resulted in a small burst of gfp expression.