The expression of viral functions is necessary for recombination of a herpesvirus (pseudorabies)

To determine whether viral functions are necessary for recombination of the pseudorabies virus genome in infected cells, we have used as a model system marker rescue at the permissive temperature (PT) and nonpermissive temperature (NPT) of a temperature sensitive mutant (tsG1) deficient in the immediate-early (180K) protein. Two restriction fragments, both of which can rescue tsG1 at the PT but only one of which encompasses the whole immediate-early gene and can complement tsG1, were compared for their ability to rescue the mutant at the NPT. Although both restriction fragments rescued the mutant with equal frequency at the PT, only the fragment which could express the immediate-early 180K protein prior to recombination, i.e. could complement tsG1, rescued the mutant at the NPT. We conclude that the expression of viral functions is necessary for high frequency recombination of the pseudorabies virus genome.