Auto-activation of the transforming viral interferon regulatory factor encoded by Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8).

Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus-8-encoded viral interferon regulatory factor (vIRF) transforms NIH3T3 cells, represses interferon signal transduction and regulates the expression of other KSHV genes. Here, we have shown that vIRF is a transcriptional activator and auto-activates its own expression. Ectopic expression of vIRF activated the vIRF promoter in KSHV-negative 293, COS7, HeLa and BJAB cell lines in a dose-dependent fashion in a reporter assay and the expression of vIRF transcripts from endogenous viral genomes in BCBL-1 and BC-1 cells latently infected with KSHV. Deletion analysis identified two cis elements, named Vac1 and Vac2, in the vIRF promoter that were responsive to vIRF activation. vIRF auto-activation via Vac1 but not Vac2 was repressed by Tis, a transcriptional silencer in the vIRF promoter. Neither Vac1 nor Vac2 contain any interferon-stimulated response element (ISRE)-like sequences and are unresponsive to induction with interferon-beta and -gamma. These results indicate that KSHV uses the mechanism of auto-activation to regulate the expression of a viral transforming protein to efficiently evade host tumour suppressor pathways.