REGULATION OF TISSUE-SPECIFIC P-ELEMENT PRE-MESSENGER-RNA SPLICING REQUIRES THE RNA-BINDING PROTEIN-PSI

Binding of a multiprotein complex to a 5' exon inhibitory element appe ars to repress splicing of the Drosophila P-element third intron (IVS3 ) in the soma. We have purified 97- and 50-kD proteins that interact s pecifically with the inhibitory element using RNA affinity chromatogra phy. Antibodies specific for the 97-kD protein relieve inhibition of I VS3 splicing in somatic extracts, providing direct evidence that inhib ition requires this protein, P-element somatic inhibitor (PSI). We ide ntify the 50-kD protein as hrp48, a protein similar to the mammalian s plicing factor hnRNP A1, and show that hrp48 recognizes specific nucle otides in a pseudo-5' splice site within the inhibitory element. The r esults indicate that PSI is an alternative splicing factor that regula tes tissue-specific splicing, probably through interactions with gener ally expressed factors such as hrp48.