Vaccinia virus-like early transcriptional control sequences flank an early gene in orf virus

The purpose of this study was to map the initiation (tsp) and termination points of transcripts arising from an open reading frame (ORF3) found in the inverted terminal repeat of the orf virus genome and also, to identify probable transcriptional control sequences. Early transcripts of approx. 0.76 kb were mapped to ORF3 and found to be transcribed toward the ends of the genome. Using the S1 nuclease and primer-extension methods, the bulk of the tsp were mapped to a position 12-13 nucleotides (nt) downstream from a sequence which resembles A + T-rich vaccinia virus early promoters. The 5' ends were 81-82 nt upstream from the first ATG in ORF3. Most of 3' ends of the transcripts mapped to a region 24-32 nt downstream from a T5NT sequence found near the ORF3 stop codon. A second transcription termination point was found 25 nt downstream from another T5NT sequence located downstream and separated by 85 nt from the first. These results infer that the A + T-rich, early transcriptional control sequences found in other poxvirus genomes have been conserved in the G + C-rich genome of orf virus.