Spontaneous BHV1 recombinants in which the gI/gE/US9 region is replaced bya duplication/inversion of the US1.5/US2 region

In a bovine herpesvirus 1 (BHV1) vaccine strain, a spontaneous BHV1 mutant (Za) was found that arose from a recombination between two isomeric forms o f the BHV 1 genome. In this Za mutant one end of the U-S region, containing part of the US1.5 gene, was found duplicated in an inverted orientation at the other end of the U-S region. Concurrently, a 2.7 kb deletion was found in Za that encompasses both the US8 (gE) and US9 gene. Analysis of the in vitro growth properties of a genetically modified BHV1gE(-) mutant showed t hat at 11 hours post infection BHV1gE(-) viruses were secreted ten times mo re efficiently than wild type virus. Using this observation we developed a protocol to enrich for spontaneous gE deletion mutants in a BHV1 field isol ate and found another mutant (Rof3) with similar properties as the Za mutan t. Rof3 has a duplication/inversion of the US 1.5 gene and part of the US2 gene and a simultaneous 3.5 kb deletion that encompasses the US7 (gI), US8 (gE) and US9 genes. The nucleotide sequences of the recombination points of both recombinants were determined and compared. No obvious sequence simila rities were found, suggesting that non-homologous recombination events led to the observed recombinations. The implications for the use of BHV1 gE del etion mutants as marker or diva vaccines are discussed.