Orf virus encodes a homolog of the vaccinia virus interferon-resistance gene E3L.

A homolog of the vaccinia virus (VAC) interferon resistance gene E3L has been discovered in orf virus strain NZ-2, a parapoxvirus that infects sheep, goats and humans. The gene is located 20 kb from the left terminus of the orf virus genome and is transcribed towards this terminus. RNase protection studies have been used to define the limits of the gene and Northern analysis revealed that it is expressed early in infection. The predicted amino acid sequence of the orf virus protein shares 31% identity (57% similarity) with the VAC E3L protein. Four of the six residues identified as being essential to dsRNA binding in the vaccinia virus protein are conserved in the orf virus protein whilst the other two amino acid changes are conservative substitutions. The orf virus gene has been sequenced in two other orf virus strains which vary markedly in their ability to produce experimental lesions in vivo. Their predicted protein sequences vary by less than 3% from the NZ-2 protein. The recombinant orf virus protein, expressed as a fusion protein in E. coli, bound double-stranded (ds)RNA but not dsDNA, single-stranded (ss)DNA or ssRNA . This is the first demonstration of a VAC E3L-like gene encoded by a parapoxvirus.