The complete genomic DNA sequence of the highly attenuated
vaccinia strain modified
vaccinia Ankara (MVA) was determined. The genome of MVA is 178 kb in length, significantly smaller than that of the
vaccinia Copenhagen genome, which is 192 kb. The 193 open reading frames (ORFs) mapped in the MVA genome probably correspond to 177 genes, 25 of which are split and/or have suffered mutations resulting in truncated
proteins. The left terminal genomic region of MVA contains four large deletions and one large insertion relative to the Copenhagen strain. In addition, many ORFs in this region are fragmented, leaving only eight genes structurally intact and therefore presumably functional. The inserted
DNA codes for a cluster of genes that is also found in the
vaccinia WR strain and in cowpox virus and includes a highly fragmented gene homologous to the cowpox virus host range gene, providing further evidence that a
cowpox-like virus was the ancestor of
vaccinia. Surprisingly, the central conserved region of the genome also contains some fragmented genes, including ORF F5L, encoding a major
membrane protein, and ORFs F11L and O1L, encoding
proteins of 39.7 and 77.6 kDa, respectively. The right terminal genomic region carries three large deletions all classical poxviral immune evasion genes and all
ankyrin-like genes located in this region are fragmented except for those encoding the
interleukin-1 beta receptor and the 68-kDa
ankyrin-like
protein B18R. Thus, the attenuated phenotype of MVA is the result of numerous mutations, particularly affecting the host interactive
proteins, including the
ankyrin-like genes, but also involving some structural
proteins.