Implication of the product of the bovine herpesvirus type 1 UL25 gene in capsid assembly.

The UL25 ORF of bovine herpesvirus type 1 (BHV-1) was demonstrated recently to represent a gene encoding a 63 kDa viral protein. To investigate the role of this gene in virus replication, a BHV-1 UL25 deletion mutant was constructed. Although the UL25 mutant synthesizes late viral proteins and viral DNA, it fails to produce virus progeny in cells that do not express the UL25 gene, demonstrating that the UL25 protein is essential for the replicative cycle of BHV-1. Moreover, Southern blotting analyses of HindIII-digested DNA from infected non-complementing cells probed with the leftward terminal fragment of the BHV-1 linear genome revealed that the cleavage of the viral DNA produced is not impaired. However, the packaging of this cleaved DNA is compromised severely, since only few full C capsids were observed in infected non-complementing cells by transmission electron microscopy.