The relationship between the SV40 genome and viral mRNA.

Studies of SV40 DNA and mRNA sequences have revealed a surprising complexity in the biogenesis of SV40 RNA. SV40 late mRNA shows heterogeneity in 5' ends, as well as internal splicing patterns. There may be coupling between different splices within the mRNA precursors and splicing is not strictly sequential. Translation may initiate at the second or third downstream AUG triplet, rather than at the first initiation codon of the mRNA. The possibility exists that translation may initiate on the same mRNA at more than one initiation codon. The DNA in proximity to the major 5' ends of late mRNA is not essential for transcription. Deletion mutants removing DNA encoding the segments of the leader sequence of late SV40 mRNA show effects on preferred transcription initiation sites both upstream and downstream. This suggests that relatively long stretches of DNA may play a role in determining the sites of transcription initiation.