Deletion of the UL21 gene in Pseudorabies virus results in the formation of DNA-deprived capsids: an electron microscopy study

We studied the morphogenesis of three pseudorabies virus mutants lacking pa rts of the gene homologous to the UL21 gene of the herpes simplex virus typ e 1. The mutants were examined in an SK-6 cell-line, in an SK-6 cell-line e xpressing the UL21 gene product, in porcine lung alveolar macrophages (PLAM ) and in porcine nasal mucosa explants. Although on SK-6 cells and PLAM, th e virus-assembly and egress of mutant virus M155, lacking almost the entire UL21 gene, was similar to that of the rescued PRV mutant, M155 producing v irions containing little or no DNA (A-type particles). Virus mutants M133 a nd M134 (lacking 23 and 232 amino acids respectively) produced more C-type particles. In SK-6 cells stably expressing the UL21-encoded protein, all mu tants produced C-type particles. All mutants produced C-type particles in n asal mucosa explants, indicating that the UL 21-gene product is not essenti al for virus production in porcine tissue. These results support and extend previous work that indicated a role for the UL21 encoded protein in the pa ckaging of newly replicated viral DNA.