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Radioactively labeled vaccinia, cowpox and Shope fibroma virions free from any detectable contamination with host cell protein, were dissociated into their constituent polypeptides, and these were then analyzed by SDS-polyacrylamide gel electrophoresis and autoradiography. The profiles of constituent polypeptide bands of four strains of vaccinia virus (IHD-W, IHD-J, Lister and DIs) were almost the same, except that a polypeptide of about 41,000 daltons was not detectable in the autoradiogram of strain IHD-W which has no hemagglutinin. The profile of polypeptide bands of cowpox virions was also almost the same as that of vaccinia virions, except for several polypeptides of about 40,000 to 50,000 daltons, but the profile of Shope fibroma virions differed considerably from that of vaccinia or cowpox virions.

The presence of cellular material in purified preparations of Shope fibroma virus (SFV) and two orthopoxviruses (vaccinia and Indiana), was investigated. Mitochondria were observed in purified preparations of SFV by electron microscopy and mitochondrial (mt) DNA was identified in restricted viral DNA by Southern blot hybridization with cloned mouse mt DNA. Mitochondrial DNA was also detected in vaccinia and SFV DNAs extracted from purified virions treated with DNase I followed by core isolation. The viral and mt DNAs could be separated on the basis of their size by agarose gel electrophoresis, but not by their buoyant density by centrifugation in cesium chloride gradients. These findings led us to re-examine previously reported results showing some homology between SFV, a leporipoxvirus and Indiana, an Orthopoxvirus (Berkowitz and Pogo, Virology 142, 437-440, 1985) using cloned fragments of SFV DNA instead of the entire viral DNA. The results indicated that cross-hybridization between SFV and Indiana DNAs was due in part to mt DNA but they also revealed an unrecognized region of homology between the two poxvirus genera.