MUTATIONS IN THE SMALL-SUBUNIT OF THE DROSOPHILA U2AF SPLICING FACTORCAUSE LETHALITY AND DEVELOPMENTAL DEFECTS

The essential eukaryotic pre-mRNA splicing factor U2AF (U2 small nucle ar ribonucleoprotein auxiliary factor) is required to specify the 3' s plice site at an early step in spliceosome assembly. U2AF binds site-s pecifically to the intron polypyrimidine tract and recruits U2 small n uclear ribonucleoprotein to the branch site. Human U2AF (hU2AF) is a h eterodimer composed of a large (hU2AF(65)) and small (hU2AF(35)) subun it. Although these proteins associate in a tight complex, the biochemi cal requirement for U2AF activity can be satisfied solely by the large subunit, The requirement for the small subunit in splicing has remain ed enigmatic, No biochemical activity has been found for hU2AF(35) and it has been implicated in splicing only indirectly by its interaction with known splicing factors, In the absence of a biochemical assay, w e have taken a genetic approach to investigate the function of the sma ll subunit in the fruit fly Drosophila melanogaster. A cDNA clone enco ding the small subunit of Drosophila U2AF (dU2AF(38)) has been isolate d and sequenced, The dU2AF(38) protein is highly homologous to hU2AF(3 5) containing a conserved central arginine- and serine-rich (RS) domai n, A recessive P-element insertion mutation affecting dU2AF(38) causes a reduction in viability and fertility and morphological bristle defe cts. Consistent with a general role in splicing, a null allele of dU2A F(38) is fully penetrant recessive lethal, like null alleles of the Dr osophila U2AF large subunit.