DNA from representative strains of rabbitpox, vaccinia, monkeypox, variola, cowpox and ectromelia viruses was analysed by cleavage with restriction endonucleases dIII, 1 or 1. Genome mol. wt. vary from about 120 × 10 for rabbitpox to about 145 × 10 for cowpox. Physical maps of cleavage sites are similar and characteristic for strains of the same type. The distribution of dIII sites suggests that an internal region of mol. wt. about 30 × 10 is highly conserved between genomes although some type-specific differences occur within this region, especially with strains of ectromelia virus. Conservation of internal sequences is less marked following analysis with 1 although cleavages within this central region of particular genomes appear to represent a subset of preferred sites. Endonuclease 1 cleaves exceptionally infrequently and distinguishes variola, monkeypox, vaccinia, cowpox or ectromelia viruses. Type specific differences result largely from extensive, near terminal variations in length and sequence.

Representative genomes have rapidly renaturing terminal restriction fragments confirming the presence of near terminal, covalent cross-links. Terminal restriction fragments from the same or different genomes generally cross hybridize indicating the presence of near terminal repetitions of mol. wt. up to 6 × 10 and which share at least a subset of common sequences. Variola strains however, appear to lack such sequences from one specific terminus which maps shorter than that of related viruses.


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