Analysis of a bovine herpesvirus 1 recombinant virus that does not express the blCP0 protein

Bovine herpesvirus 1 (BHV-1) infected-cell protein 0 (bICPO) stimulates productive infection by activating viral gene expression. In this study, an attempt was made to construct a recombinant virus with point mutations in the C3HC4 zinc RING finger of blCP0, as this domain is necessary for activating viral transcription and productive infection. A virus was identified in bovine cells that induced small clusters of infected cells resembling a small plaque. Instead of the expected mutations within the zinc RING finger, this virus contained a point mutation within the initiating ATG of blCP0, a point mutation two bases downstream from the ATG mutation and deletion of flanking plasmid sequences used for homologous recombination. The blCP0 mutant was rescued with wild-type (wt) blCP0 sequences and the blCP0-rescued virus produced wt plaques. The blCP0-rescued virus and wt BHV-1, but not the mutant, expressed the blCP0 protein during productive infection of bovine cells, suggesting that the mutant virus was a null mutant. Consequently, the mutant was designated the blCP0 null mutant. Infection of bovine cells with the blCP0 null mutant resulted in at least 100-fold lower virus titres, indicating that blCP0 protein expression is important, but not required, for virus production. When bovine cells infected with the blCP0 null mutant virus were subcultured, the cells continued to divide, but viral DNA could be detected after more than 35 passages, suggesting that the blCP0 null mutant induced a persistent-like infection in bovine cells and that it may be useful for generating additional blCP0 mutants.