Species-specific differences in the organization of genes encoding kelch-like proteins of orthopoxviruses pathogenic for humans

Identification of 10 cowpox virus proteins that are necessary for induction of hemorrhagic lesions (red pocks) on chorioallantoic membranes

Cowpox viruses (CPXV) cause hemorrhagic lesions ("red pocks") on infected chorioallantoic membranes (CAM) of embryonated chicken eggs, while most other members of the genus Orthopoxvirus produce nonhemorrhagic lesions ("white pocks"). Cytokine response modifier A (CrmA) of CPXV strain Brighton Red (BR) is necessary but not sufficient for the induction of red pocks. To identify additional viral proteins involved in the induction of hemorrhagic lesions, a library of single-gene CPXV knockout mutants was screened. We identified 10 proteins that are required for the formation of hemorrhagic lesions, which are encoded by CPXV060, CPXV064, CPXV068, CPXV069, CPXV074, CPXV136, CPXV168, CPXV169, CPXV172, and CPXV199. The genes are the homologues of F12L, F15L, E2L, E3L, E8R, A4L, A33R, A34R, A36R, and B5R of vaccinia virus (VACV). Mutants with deletions in CPXV060, CPXV168, CPXV169, CPXV172, or CPXV199 induced white pocks with a comet-like shape on the CAM. The homologues of these five genes in VACV encode proteins that are involved in the production of extracellular enveloped viruses (EEV) and the repulsion of superinfecting virions by actin tails. The homologue of CPXV068 in VACV is also involved in EEV production but is not related to actin tail induction. The other genes encode immunomodulatory proteins (CPXV069 and crmA) and viral core proteins (CPXV074 and CPXV136), and the function of the product of CPXV064 is unknown.

IMPORTANCE

It has been known for a long time that cowpox virus induces hemorrhagic lesions on chicken CAM, while most of the other orthopoxviruses produce nonhemorrhagic lesions. Although cowpox virus CrmA has been proved to be responsible for the hemorrhagic phenotype, other proteins causing this phenotype remain unknown. Recently, we generated a complete single-gene knockout bacterial artificial chromosome (BAC) library of cowpox virus Brighton strain. Out of 183 knockout BAC clones, 109 knockout viruses were reconstituted. The knockout library makes possible high-throughput screening for studying poxvirus replication and pathogenesis. In this study, we screened all 109 single-gene knockout viruses and identified 10 proteins necessary for inducing hemorrhagic lesions. The identification of these genes gives a new perspective for studying the hemorrhagic phenotype and may give a better understanding of poxvirus virulence.

Genome comparison of a nonpathogenic myxoma virus field strain with its ancestor, the virulent Lausanne strain

One of the best-studied examples of host-virus coevolution is the release of myxoma virus (MV) for biological control of European rabbits in Australia and Europe. To investigate the genetic basis of MV adaptation to its new host, we sequenced the genome of 6918, an attenuated Spanish field strain, and compared it with that of Lausanne, the strain originally released in Europe in 1952. Although isolated 43 years apart, the genomes were highly conserved (99.95% identical). Only 32 of the 159 MV predicted proteins revealed amino acid changes. Four genes (M009L, M036L, M135R, and M148R) in 6918 were disrupted by frameshift mutations.

Poxviruses: past, present and future

The analysis of poxvirus genomes is complex, in part, because of their size (130-360 kb) and the fact that gene content is variable; a common set of 49 genes has been found in all sequenced poxviruses and an additional 41 genes are also present in all sequenced orthopoxviruses. As a group, poxviruses have a very broad range of eukaryotic hosts (including mammals, birds, reptiles and insects) and many poxvirus genes are associated with blocking host anti-viral responses. One consequence of this is that many poxvirus genes are not essential for growth in tissue culture and that extensive passaging in vitro results in the accumulation of mutations, including deletions that result in loss of gene function. Here, we review various comparative analyses of the poxviruses including gene prediction, gene conservation and function, genome organization, and poxvirus taxonomy and evolution.