In situ structural organization of encapsidated and unencapsidated Ectromelia virus DNA

The structural organization of Ectromelia virus DNA in infected mouse liver cells has been studied by using thin sections stained with the Feulgen-like osmium-ammine reaction. We found that in the cytoplasmic factories, free viral DNA was structured into completely extended filaments 2-3 nm thick. Viral DNA in immature virions, however, appeared to have a structural organization that superimposed that of eukaryotic chromatin. This was constituted by roundish subunits, with a diameter of 11-13 nm, composed of a DNA ring encircling an unstained inner core. The mature virion was composed of the same type of subunits, which were arranged in threads twisted into a figure 8 configuration. The distribution of basic proteins was also investigated with the acrolein silver-methenamine technique. In the viral particles only nucleoids were stained; a uniformly distributed positive reaction was observed in the cytoplasmic factories.