The localization and the function of cellular transcription factors in vaccinia virus

Vaccinia virus, a family of poxviridae, is a large DNA virus. The life cycle of vaccinia virus occurs in the cytoplasm of host cells. Vaccinia virus is regulated by the temporal transcription at the early, intermediate and late stages. Gene expression of each stage activates the transcription of the next stage. The lack of any vaccinia virus-encoded factors interacting with intermediate and late classes of promoters suggests that host cellular proteins may participate in the regulation of vaccinia viral transcription. The recruitment of host nuclear proteins in vaccinia virus intermediate transcription was investigated by localization and functional studies. Viral DNA replication before intermediate transcription was shown to be necessary and sufficient for the cytoplasmic translocation of YY1, a nuclear transcription factor. It was also demonstrated that the DNA binding domain of YY1 was important for its translocation into the cytoplasm after infection. Experiments with transport inhibitors showed that YY1 is not transported by either passive diffusion or a Crm1-mediated system. The overexpression and RNA interference studies of YY1 implied that the translocated YY1 represses the vaccinia virus intermediate transcription. It was previously described that the overexpression of TBP activates vaccinia virus intermediate transcription. A TBP RNA interference study supported overexpression results. Based on the above results and YY1 binding site mutation experiments, a replacement model of viral intermediate transcription is suggested. In this model, YY1 competes with a yet-to-be-identified nuclear activator(s) for the binding site on vaccinia virus intermediate promoter