The SPI-1 gene of rabbitpox virus determines host range and is required for hemorrhagic pock formation

Wild-type rabbitpox virus (RPV) and cowpox virus (CPV) produce red hemorrhagic lesions or pocks upon infection of the chicken chorioallantoic membrane (CAM) of 11-day-old embryonated chicken eggs. However, white, nonhemorrhagic pock variants arise spontaneously within wild-type (wt) populations of either virus at a frequency of about 1%, reflective of complex deletions/rearrangements in the termini of the viral DNA. A subpopulation of the RPV white-pock mutants fail to plaque on pig kidney (PK-15) cells and are referred to as host-range (hr) mutants. In the case of CPV, white-pock formation has been linked to mutations in the SPI-2 (crmA) gene. We show that five spontaneous RPV white-pock host-range mutants (RPV mu hr8sm, RPV mu hr23, RPV mu hr28, RPV mu hr30, and RPV mu hr31) each contain a SPI-2 (crmA) gene and express the crmA protein but lack instead a functional SPI-1 gene. Two other spontaneous RPV white-pock mutants, RPV mu 9 and RPV mu 12, which plaque on PK-15 cells (nonhost-range mutants) contain and express a SPI-1 gene but lack instead a functional SPI-2 gene. Targeted disruption of either the SPI-1 or SPI-2 genes of wtRPV, but only the SPI-2 gene of wtCPV, generates mutants which produce white pocks. The RPV delta SPI-1 mutant fails to plaque on PK-15 or human A549 cells, whereas the RPV delta SPI-2 mutant has a normal host range. No changes in host range compared to wtCPV for either the CPV delta SPI-1 or CPV delta SPI-2 mutants were noted. These differences in phenotypes observed between the two viruses may be reflective of either small sequence variations between the highly conserved SPI-1 or SPI-2 genes or the aggregate phenotypes provided by the other remaining genes.