Conservation of gene structure and arrangement between vaccinia virus and orf virus.

A 3.3-kb BamHI fragment from the center of the orf virus (OV) NZ2 genome has been sequenced, revealing three major open reading frames (ORFs) with homology to vaccinia virus (VAC) genes. These ORFs have been designated F2L, F3R, and F4R and the proteins they encode were found to be homologous to VAC genes H4L (RNA polymerase-associated protein RAP94), H5R (35-kDa virion envelope antigen) and H6R (topoisomerase), respectively. The OV ORFs are arranged on the genome in an almost identical manner to their VAC counterparts revealing the common evolutionary origin of the two viruses despite the extreme difference in their G+C content. Like its VAC counterpart, F3R was shown to be transcribed early and late during infection. S1 and primer extension analysis located the 5' ends of F3R early transcripts to a position 15-16 nt and 5-10 nt, respectively, downstream from an AT-rich sequence resembling a VAC early promoter. The 5' ends of F3R late transcripts were located to an A within the sequence 5'-TAAAG, 41 nt downstream from the early promoter and 17 nt upstream from the initiation codon. S1 analysis of F2L, which is transcribed only late in infection, revealed transcripts initiating from within the sequence 5'-TAAATG. No transcriptional start point could be detected for F4R but the VAC late transcriptional initiation sequence TAAAT was found close to the predicted translational start point. Another late promoter-like sequence, 5'-TAAATG, was found at the 3' end of F2L. This preceded a short ORF tentatively designated as F1L and predicted to be the beginning of a homologue of VAC H3L.