Identification of a bidirectional splicing enhancer: Differential involvement of SR proteins in 5 ' or 3 ' splice site activation

The adenovirus E1A pre-mRNA undergoes alternative splicing whose modulation occurs during infection, through the use of three different 5' splice site s and of one major or one minor 3' splice site. Although this pre-mRNA has been extensively used as a model to compare the transactivation properties of SR proteins, no cis-acting element has been identified in the transcript sequence; Here we describe the identification and the characterization of a purine-rich splicing enhancer, located just upstream of the 12S 5' splice site, which is formed from two contiguous 9-nucleotide (nt) purine motifs (Pu1 and Pu2). We demonstrate that this sequence is a bidirectional splicin g enhancer (BSE) in vivo and in vitro, because it activates both the downst ream 12S 5' splice site through the Pul motif and the upstream 216-nt inter vening sequence (IVS) 3' splice site through both motifs. UV cross-linking and immunoprecipitation experiments indicate that the BSE interacts with se veral SR proteins specifically, among them 9G8 and ASF/SF2, which bind pref erentially to the Pul and Pu2 motifs, respectively. Interestingly, we show by in vitro complementation assays that SR proteins have distinct transacti vatory properties. In particular, 9G8, but not ASF/SF2, or SC35, is able to strongly activate the recognition of the 12S 5' splice site in a BSE-depen dent manner in wild-type E1A or in a heterologous context, whereas ASF/SF2 or SC35, but not 9G8, activates the upstream 216-nt IVS splicing. Thus, our results identify a novel exonic BSE and the SR proteins which are involved in its differential activity.