Activities of heterodimers composed of DNA-binding- and transactivation-deficient subunits of the herpes simplex virus regulatory protein ICP4.

Two mutant strains (vi12 and vi13) of herpes simplex virus that contain insertion mutations in the sequences that encode the DNA-binding domain of viral regulatory protein ICP4 were generated. Both mutations disrupted specific DNA binding and resulted in transcriptionally inactive molecules; however, the ability of the mutant proteins to form dimers was retained. The mutant proteins formed heterodimers with an ICP4 deletion mutant (X25) that is nonfunctional but retains the ability to bind to consensus sites. Significantly elevated levels of early (E or beta) and "leaky late" (beta gamma or gamma 1) gene expression were observed upon coexpression of the insertion mutant and X25 ICP4 polypeptides. While the heterodimers composed of the vi13 and X25 peptides possessed DNA-binding activity, those composed of vi12 and X25 did not, indicating that DNA binding by the heterodimers may not be required for restored activity. Despite significant levels of early gene expression and viral DNA synthesis in vi12-infected X25 cells, true late (gamma 2) mRNA was not synthesized. This indicates that the structural requirements for ICP4 induction of different classes of viral genes may be different.