Serine-arginine (SR)-rich splicing factors have an exon-independent function in pre-mRNA splicing

Two distinct functions have been proposed for the serine-arginine (SR)-rich family of splicing factors. First, SR proteins are essential splicing fact ors and are thought to function by mediating protein-protein interactions w ithin the intron during spliceosome assembly. Second, SR proteins bind to e xonic enhancer sequences and recruit spliceosome components to adjacent int rons. The latter activity is required for splice-site recognition and alter native splicing. Until now it has not been possible to determine whether th e requirement for SR proteins in the basic splicing reaction is a secondary consequence of their exon-dependent recruitment function. Here we show tha t RNA substrates containing only 1 nt of exon sequence can undergo the firs t step of the splicing reaction in vitro and that this activity requires SR proteins. Thus, we provide direct evidence that SR proteins have both exon -independent and exon-dependent functions in pre-mRNA splicing.