Vaccinia virus encodes a family of genes with homology to serine proteinase inhibitors.

Nucleotide sequencing of a region of the vaccinia virus genome proximal to the right inverted terminal repeat (ITR) identified two open reading frames (ORFs) encoding proteins of 39K and 40K with amino acid homology to each other, to another vaccinia virus gene near the opposite end of the virus genome and to the superfamily of serine proteinase inhibitors (serpins). Serpins have now been found in poxviruses from the genera orthopox (cowpox and vaccinia viruses), leporipox (myxoma virus) and avipox (fowlpox virus). One of the vaccinia virus serpins identified here (B13R) shares 92% amino acid identity with the serpin from cowpox virus and 46% and 19% identity with vaccinia serpins B24R and K2L, respectively. The amino acid sequence of B13R reported here differs at 11 positions from a recently reported sequence and contains an additional three internal residues. The serpin genes near the right ITR are separated by 8 kb of DNA. Both genes contain early transcriptional termination signals just downstream of the ORFs and are transcribed in a rightward direction towards the end of the genome. Analysis of mRNAs from virus-infected cells demonstrated that all three vaccinia virus serpin genes are transcribed early during infection. The amino acid sequences at the active sites of these serpins suggest that they may inhibit serine proteinases of differing biochemical specificities. The possible functions of these genes are discussed.