Fine-mapping of sequences that suppress splicing in the E3 complex transcription unit of adenovirus

Complex transcription units encode multiple mRNAs that arise by alternative processing of a common pre-mRNA. Some mechanism must exist that maintains the balance among the various mRNAs. The E3 complex transcription unit of adenovirus encodes four major mRNAs (termed a, c, f, and h) and several minor mRNAs (d, e, i). mRNAs a and c account for about one-half of E3 mRNAs. Using virus deletion mutants we have identified a region (termed Region I) which, when deleted, leads to the formation of doubly spliced mRNAs f and h at the expense of singly spliced mRNAs a and c. Region I consists of at most 129 nucleotides. Deletions as small as 24 nucleotides markedly abolished the splice-suppressing function of Region I. We propose that Region I normally functions to suppress splicing in the E3 pre-mRNA and thereby allow mRNA a and c to be formed in abundance. We have also identified another region, designated as Region IA, that appears to specifically suppress splicing to form the doubly-spliced minor mRNAs d and e. Neither Region I nor Region IA include a conventional splice site or cleavage/polyadenylation site. We speculate that Region I and Region IA probably suppress splicing by binding a trans-acting factor.