Tumorigenic poxviruses: Transcriptional mapping of the terminal inverted repeats of shope fibroma virus

A composite transcriptional map for the entire 12.4-kb terminal inverted repeat (TIR) region of the Shope fibroma virus (SFV) genome has been determined. Northern blotting and S1-nuclease mapping were used to determine the regions which are transcribed, their temporal relationships, as well as the transcriptional initiation sites. Sequences representing the entire TIR are transcribed into poly(A)⁺ mRNA at both early and late times in the infection. Fifteen transcriptional initiation sites were mapped, 12 within the TIRs and 3 within the unique sequences close to the junction between the right TIR and the unique internal sequences. Ten of the 12 transcriptional initiation sites within the TIR and 2 of the 3 sites outside the right TIR correspond to the 5′-ends of the major open reading frames (ORFs) Ti to T9 plus the SFV growth factor gene. The 3 other initiation sites map within ORFs but near potential start codons for shorter polypeptides. All the expressed ORFs are tandemly arranged and transcribed toward the hairpin terminus. At early times during SFV infection of cultured rabbit cells, transcription of each ORF gives rise to a transcript of distinct size, while at late times termination of transcription is imprecise and substantial read-through into downstream sequences occurs. These results are discussed in light of recent observations on the related recombinant leporipoxvirus, malignant rabbit fibroma virus, which suggest that one or more gene products from this region of the SFV genome are implicated in viral tumorigenicity.