POLY(U)-BINDING PROTEIN INHIBITS DROSOPHILA PRE-5 S-RNA 3'-EXONUCLEASE DIGESTION

A approximately 50-kDa protein binds specifically to the 3' terminus o f 135-nucleotide Drosophila pre-5 S RNA. Unlabeled poly(U) competes ou t protein binding and stimulates the activity of a 3'-exonuclease, whi ch eventually degrades the substrate to 120 nucleotides, the size of m ature 5 S RNA. In its RNA binding and UV cross-linking properties, the endogenous poly(U)-binding protein resembles human La, an autoantigen that binds the U>3 3' ends of vertebrate RNA polymerase III primary t ranscripts. This protein appears to inhibit a 3' exonuclease and could protect 5 S RNA for faithful processing and transport.