A novel intergenic site for integration and expression of foreign genes in the genome of pseudorabies virus.

Restriction enzyme analysis of DNA of a number of Pseudorabies virus (PRV) single plaque isolates revealed in several cases the existence of a unique EcoRI cleavage site, which has not been observed in PRV DNA before. This EcoRI site was mapped to the right end of the unique long region of the PRV genome, in BamHI-fragment 6. Sequence analysis of this region demonstrated the presence of an 11 bp tandem repeat in variable copy numbers in different PRV strains, suggesting the creation of the EcoRI recognition site by a recombinational event. The occurrence of variable reiterations and Northern blot analysis indicated an intergenic region. We therefore, used this site for integration and expression of heterologous DNA (the multiple cloning site of phage M13 and the E. coli lacZ gene). Viable PRV recombinants could be obtained which showed no detectable differences in virus growth in vitro compared to wild-type PRV. The novel insertion site can be used for the construction of PRV recombinants expressing foreign genes without apparent impairment of PRV genes.