Antagonism between RSF1 and SR proteins for both splice-site recognition in vitro and Drosophila development

Specific recognition of splice sites within metazoan mRNA precursors (pre-m RNAs) is a potential stage for gene regulation by alternative splicing. Spl icing factors of the SR protein family play a major role in this regulation , as they are required for early recognition of splice sites during spliceo some assembly. Here, we describe the characterization of RSF1, a splicing r epressor isolated from Drosophila, that functionally antagonizes SR protein s, Like the latter, RSF1 comprises an amino-terminal RRM-type RNA-binding d omain, whereas its carboxy-terminal part is enriched in glycine (G), argini ne (R), and serine (S) residues (GRS domain). RSF1 induces a dose-sensitive inhibition of splicing for several reporter pre-mRNAs, an inhibition that occurs at the level of early splicing complexes formation. RSF1 interacts, through its GRS domain,with the RS domain of the SR protein SF2/ASF and pre vents the latter from cooperating with the U1 small nuclear ribonucleoprote in particle (U1 snRNP) in binding pre-mRNA. Furthermore, overproduction of RSF 1 in the fly rescues several developmental defects caused by overexpres sion of the splicing activator SR protein B52/SRp55, Therefore, RSF1 may co rrespond to the prototypical member of a novel family of general splicing r epressors that selectively antagonize the effect of SR proteins on 5' splic e-site recognition.