Ribonucleotide reductase-deficient mutants of pseudorabies virus are avirulent for pigs and induce partial protective immunity.

We have mutagenized and mapped the gene encoding the large subunit of ribonucleotide reductase (RR1) in pseudorabies virus (PRV; synonyms Aujeszky's disease virus, suid herpesvirus type 1). PRV strains carrying an oligonucleotide that leads to termination of translation of the RR1 gene are avirulent for mice. We subsequently constructed a PRV strain carrying a deletion in the RR1 gene and also a PRV strain carrying both the deletion in the RR1 gene and a deletion in the glycoprotein g1 gene, which is a marker for PRV virulence. Both PRV strains were assayed for virulence and immunogenicity in pigs, the natural host for PRV. In contrast to a marker-rescued PRV strain, these RR1-deleted mutants were avirulent, were shed in very low titres in the oropharyngeal fluid by the animals, and induced low titres of neutralizing antibodies. However, protection against clinical signs after infection with virulent PRV was induced by both RR1-deleted mutants. The relative importance of viral RR and thymidine kinase enzymes for deoxynucleotide synthesis in viral replication is discussed. In addition, we discuss the potential use of RR as a target for anti-herpesviral drugs and the use of PRV strains, deleted for the RR1 gene, as vaccine strains.