Genomic comparison of an avirulent strain of orf virus with that of a virulent wild type isolate reveals that the orf virus G2L gene is non-essentialfor replication

The genomic structure of two strains of orf virus (OV), a field isolate (MR I-Scab) which has never been passaged in cell culture, and a multiple-passa ge cell culture-adapted strain (Orf-11) were compared. The Orf-11 genome is similar to8.0 kb longer than that of the MRI-Scab due to a duplication of the right-hand end. The duplicated region has been translocated to the left -hand end of the genome with a loss of sequence from that end. The lost seq uence contains three complete genes, namely E2L, E3L and G1L and 80% of a f ourth gene, namely G2L. The sequence lost from G2L in Orf-11 has been repla ced by a region of unrelated sequence, encoding 98 amino acids. Northern an alysis shows that mRNA is expressed from this "new" gene. The two viruses w ere also compared for in vivo virulence and ability to protect against subs equent OV challenge. In vivo, the field isolate was fully virulent and conf erred good protection against challenge, whereas the cell culture-adapted v irus produced only mild lesions and reduced protection against challenge.