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The post‐translational modification of proteins by covalent attachment of ubiquitin occurs in all eukaryotes by a multi‐step process. A family of E2 or ubiquitin conjugating (UBC) enzymes catalyse one step of this process and these have been implicated in several diverse regulatory functions. We report here the sequence of a gene encoded by African swine fever virus (ASFV) which has high homology with UBC enzymes. This ASFV encoded enzyme has UBC activity when expressed in Escherichia coli since it forms thiolester bonds with [125I]ubiquitin in the presence of purified ubiquitin activating enzyme (E1) and ATP, and subsequently transfers [125I]ubiquitin to specific protein substrates. These substrates include histones, ubiquitin and the UBC enzyme itself. The ASFV encoded UBC enzyme is similar in structure and enzyme activity to the yeast ubiquitin conjugating enzymes UBC2 and UBC3. This is the first report of a virus encoding a functionally active UBC enzyme and provides an example of the exploitation of host regulatory mechanisms by viruses.

African swine fever virus is an icosahedral double-stranded DNA virus which replicates in the cytoplasm of porcine monocytic cells. Progeny virus particles, like poxviruses and iridoviruses, are assembled in cytoplasmic inclusions, termed virus factories. The formation of these structures in susceptible cells infectedin vitrowith pathogenic and attenuated ASFV isolates has been studied by semiquantitative and qualitative electron microscopy. Recognizable virus factory elements were detected by 8 hr postinfection (hpi) and increased significantly in profile area between 12 and 18 hpi. The number of virus particles associated with the virus factories also increased significantly between 12 and 24 hpi. These included particles with (“full”) and without (“empty”) a nucleo-protein core. Similar results were obtained for both pathogenic (Malawi) and attenuated (Uganda) virus isolates, but the replication of pathogenic virus in the macrophage was more efficient than that of the attenuated virus in a porcine kidney cell line, where a relatively higher percentage of defective “empty” particles were observed. Analysis of virus factory formation was also done directly on lung and liver tissue from a pig infected with the highly pathogenic Malawi virus isolate. Thein vivodata for virus factory area and particle number in both pulmonary intravascular macrophages and liver Kupffer cells at Day 4 postinfection were similar to those observedin vitro.As expected, using postembedding immunoelectron microscopy, DNA (both cellular and viral) was detected in the cell nucleus, cytoplasmic virus, and mature extracellular virus. More interestingly, DNA was absent in the cytoplasm, but readily observed in virus factories. This DNA, which we presume to be viral, was found in close association with membrane-like material and “empty” particles, suggesting that the virus DNA may enter a partially formed capsid which is then sealed in order to develope into a fully assembled particle. According to this hypothesis, ASFV morphogenesis involves the initial formation of “empty” particles within the virus factory. The adjacently formed nucleo-protein material is then inserted into the “empty” particles, as has been described for poxviruses. These particles then mature in to the “full” particles and leave the virus factory as a prelude to release from the cell by budding.

The post‐translational modification of proteins by covalent attachment of ubiquitin occurs in all eukaryotes by a multi‐step process. A family of E2 or ubiquitin conjugating (UBC) enzymes catalyse one step of this process and these have been implicated in several diverse regulatory functions. We report here the sequence of a gene encoded by African swine fever virus (ASFV) which has high homology with UBC enzymes. This ASFV encoded enzyme has UBC activity when expressed in Escherichia coli since it forms thiolester bonds with [125I]ubiquitin in the presence of purified ubiquitin activating enzyme (E1) and ATP, and subsequently transfers [125I]ubiquitin to specific protein substrates. These substrates include histones, ubiquitin and the UBC enzyme itself. The ASFV encoded UBC enzyme is similar in structure and enzyme activity to the yeast ubiquitin conjugating enzymes UBC2 and UBC3. This is the first report of a virus encoding a functionally active UBC enzyme and provides an example of the exploitation of host regulatory mechanisms by viruses.