CONSERVATION AND VARIATION IN ORTHOPOXVIRUS GENOME STRUCTURE

Orthopoxvirus DNA from representative strains of rabbitpox, vaccinia, monkeypox, variola, cowpox and ectromelia viruses was analysed by cleavage with restriction endonuclease HindIII, Xho1 or Sma1. Genome mol. wt. vary from about 120 x 106 for rabbitpox to about 145 x 106 for cowpox. Physical maps of cleavage sites are similar and characteristic for strains of the same Orthopoxvirus type. The distribution of HindIII sites suggests that an internal region of mol. wt. about 30 x 106 is highly conserved between Orthopoxvirus 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 Xho1 although cleavage within this central region of particular genomes appear to represent a subset of preferred sites. Endonuclease Sma1 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 Orthopoxvirus 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 x 106 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.