Cloning of Caenorhabditis U2AF65: an alternatively spliced RNA containing a novel exon.

The U2 small nuclear ribonucleoprotein particle (snRNP) auxiliary factor, U2AF, is an essential splicing factor required for recognition of the polypyrimidine tract and subsequent U2 snRNP assembly at the branch point. Because Caenorhabditis elegans introns lack both polypyrimidine tract and branch point consensus sequences but have a very highly conserved UUUUCAG/R consensus at their 3' splice sites, we hypothesized that U2AF might serve to recognize this sequence and thus promote intron recognition in C. elegans. Here we report the cloning of the gene for the large subunit of U2AF, uaf-1. Three classes of cDNA were identified. In the most abundant class the open reading frame is similar to that for the U2AF65 from mammals and flies. The remaining two classes result from an alternative splicing event in which an exon containing an in-frame stop codon is inserted near the beginning of the second RNA recognition motif. However, this alternative mRNA is apparently not translated. Interestingly, the inserted exon contains 10 matches to the 3' splice site consensus. To determine whether this feature is conserved, we sequenced uaf-1 from the related nematode Caenorhabditis briggsae. It is composed of six exons, including an alternatively spliced third exon interrupting the gene at the same location as in C. elegans. uaf-1 is contained in an operon with the rab-18 gene in both species. Although the alternative exons from the two species are not highly conserved and would not encode related polypeptides, the C. briggsae alternative exon has 18 matches to the 3' splice site consensus. We hypothesize that the array of 3' splice site-like sequences in the pre-mRNA and alternatively spliced exon may have a regulatory role. The alternatively spliced RNA accumulates at high levels following starvation, suggesting that this RNA may represent an adaption for reducing U2AF65 levels when pre-mRNA levels are low.