Effects of a temperature-sensitive mutation in the immediate-early gene of pseudorabies virus on class II and class III gene transcription.

The pseudorabies virus immediate-early protein activates transcription of both class II and class III genes. At present it is not known whether the activation of class II genes occurs through an activation of cellular factors or by a direct interaction of the immediate-early protein with factors or DNA. It is also not known whether the activation of class II and class III genes occurs by a similar or different mechanism. We utilized tsG, a temperature-sensitive mutation in the immediate-early gene of pseudorabies virus, to study the activation of viral genes transcribed by RNA polymerases II and III. Previous studies have shown that tsG inhibits wild-type adenovirus early gene transcription in coinfections at the nonpermissive temperature (L. T. Feldman and S. E. Ahlers, J. Virol. 57:13-17, 1986). Using this system of mixed infections as an assay, we obtained several results which allowed us to draw certain conclusions about the mode of action of the pseudorabies virus immediate-early protein (IEP). First, the tsG mutation inhibits the formation of new transcription complexes on class II genes, but does not affect transcription from preestablished transcription complexes formed in the presence or absence of the adenovirus E1A protein. Second, tsG does not inhibit transcription from class III genes, suggesting that the activation by IEP of class II and class III genes occurs by different mechanisms. Third, activation of transcription by the adenovirus E1A protein is not dominant to inhibition by tsG, suggesting that the temperature-sensitive IEP is involved in some physical interaction which actively inhibits transcription of viral genes.