The nucleotide sequence of the 3913 base pair XhoI O fragment located in an evolutionary variable region adjacent to the right end of the genome of ectromelia virus (EMV) was determined. The sequence contains two long open reading frames coding for putative
proteins of 559
amino acid residues (p65) and 344
amino acid residues (p39).
Amino acid database searches showed that p39 is closely related to vaccinia virus (VV), strain WR, B22R gene product (C12L gene product of strain Copenhagen), which belongs to the family of
serine protease inhibitors (
serpins). Despite the overall high conservation, differences were observed in the sequences of p39, B22R, and C12L in the site known to interact with
proteases in other
serpins, suggesting that the
serpins of EMV and two strains of VV may all inhibit
proteases with different specificities. The gene coding for the ortholog of p65 is lacking in the Copenhagen strain of vaccinia virus; the WR strain contains a truncated variant of this gene (B21R) potentially coding for a small
protein (p16) corresponding to the C-terminal region of p65. p65 is a new member of the family of poxvirus
proteins including vaccinia virus
proteins A55R, C2L and F3L, and a group of related
proteins of leporipoxviruses,
Shope fibroma and myxoma viruses (T6, T8, T9, M9). These
proteins are homologous to the Drosophila
protein Kelch involved in egg development. Both Kelch
protein and the related poxvirus
proteins contain two distinct domains. The N-terminal domain is related to the similarly located domains of
transcription factors Ttk, Br-C (Drosophila), and KUP (human), and GCL
protein involved in early development in Drosophila. The C-terminal domain consists of an array of four to five imperfect repeats and is related to human placental
protein MIPP. Phylogenetic analysis of the family of poxvirus
proteins showed that their genes have undergone a complex succession of duplications, and complete or partial deletions.