An exonic splicing silencer in the testes-specific DNA ligase III beta exon

Alternative pre-mRNA splicing of two terminal exons (alpha and beta ) regul ates the expression of the human DNA ligase III gene. In most tissues, the a exon is expressed. In testes and during spermatogenesis, the beta exon is used instead. The alpha exon encodes the interaction domain with a scaffol d DNA repair protein, XRCC1,while the beta exon-encoded C-terminal does not . Sequence elements regulating the alternative splicing pattern were mapped by in vitro splicing assays in HeLa nuclear extracts. Deletion of a region beginning in the beta exon and extending into the downstream intron derepr essed splicing to the beta exon, Two silencing elements were found within t his 101 nt region: a 16 nt exonic splicing silencer immediately upstream of the beta exon polyadenylation signal and a 45 nt intronic splicing silence r, The exonic splicing silencer inhibited splicing, even when the polyadeny lation signal was deleted or replaced by a 5' splice site, This element als o enhanced polyadenylation under conditions unfavourable to splicing, The s plicing silencer partially inhibited assembly of spliceosomal complexes and functioned in an adenoviral pre-mRNA context. Silencing of splicing by the element was associated with cross-linking of a 37 kDa protein to the RNA s ubstrate. The element exerts opposite functions in splicing and polyadenyla tion.