The ps/hr gene (B5R open reading frame homolog) of rabbitpox virus controls pock color, is a component of extracellular enveloped virus, and is secreted into the medium.

Wild-type rabbitpox virus (RPV) produces red hemorrhagic pocks on the chorioallantoic membranes (CAMs) of embryonated chicken eggs. Like the crmA (SPI-2) gene of cowpox virus, disruption of the RPV ps/hr gene results in a mutant which produces white pocks on the CAMs. An examination of the properties of the RPV(ps/hr) mutant in cell culture also reveals a significantly reduced host range, defined as the inability to form plaques, compared with wild-type virus. One of several cell types on which RPV(ps/hr) mutants fail to produce plaques is chicken embryo fibroblasts, cells which have been traditionally used to propagate spontaneously arising white pock mutants isolated from CAMs. The inability of the RPV(ps/hr) mutant to form plaques in chicken embryo fibroblasts correlates with a failure of a low multiplicity of infection to spread to neighboring cells and to form extracellular enveloped virus (EEV), although the formation and yields of infectious intracellular naked virus appear relatively normal. The gene product of the ps/hr gene, initially synthesized as a 45-kDa glycoprotein, is found as a component of EEV, but not intracellular naked virus, and as a smaller, secreted soluble protein of 35 kDa. Production of the secreted 35-kDa protein was found to be independent of any viral morphogenesis, suggesting two distinct pathways for release of the ps/hr gene product from the cell, i.e., as a component of the EEV particle and as a separately secreted glycoprotein.