Pseudorabies virus avirulent strains fail to express a major glycoprotein

The unique short (Us) region of the pseudorabies virus (PRV) genome, which displays high transcriptional activity during the late phase of infection and has been found to code for glycoproteins, is partially deleted in the genomes of three vaccine strains (A57, Norden, and NIA-4). This deletion is located in the SalI subfragment 7A of BamHI fragment 7. To identify possible viral gene products involved in PRV virulence, we investigated the transcriptional and translational pattern of the deleted part of the Us region. Northern blots demonstrated that one major RNA species (3.8 kilobases) transcribed from fragment 7A was missing in the vaccine strains, whereas other transcripts were altered. Radioimmunoprecipitation of in vivo-labeled PRV glycoproteins and of in vitro-translated polypeptides with hyperimmune serum and monoclonal antibodies indicated a lack of glycoprotein gI. Hybrid-selection experiments with subcloned DNA fragments confirmed the absence of gI and of a 40,000-molecular-weight polypeptide. We suggest that both viral proteins are involved in the expression of PRV virulence.