Virus deletion mutants that affect a 3' splice site in the E3 transcription unit of adenovirus 2.

Five viable virus mutants were constructed with deletions near a 3' splice site located at nucleotide 2157 in the E3 transcription unit of adenovirus 2. The mutants were examined for splicing activity at the 2157 3' splice site in vivo by nuclease-gel analysis of steady-state cytoplasmic mRNA. Splicing was not prevented by an exon deletion (dl719) that leaves 16 5'-proximal exon nucleotides intact or by intron deletions that leave 34 (dl717, dl712) or 18 (dl716) 3'-proximal intron nucleotides intact. The sequences deleted in one of these intron mutants (dl716) include the putative branchpoint site used in lariat formation during splicing. Thus, a surrogate branchpoint site apparently can be used for splicing. Another intron mutant (dl714) has a deletion that leaves 15 3'-proximal intron nucleotides intact; remarkably, this deletion virtually abolished splicing, even though the deletion is only 3 nucleotides closer to the splice site than is the deletion in dl716 which splices normally. The three nucleotides deleted in dl714 that are retained by dl716 are the sequence TGT. The TGT sequence is located on the 5' boundary of the pyrimidine-rich region upstream of the nucleotide 2157 3' splice site. Such pyrimidine-rich regions are ubiquitous at 3' splice sites. Most likely, the TGT is required for splicing at the nucleotide 2157 3' splice site. The TGT may be important because of its specific sequence or because it forms the 5' boundary of the pyrimidine-rich region.