Transcription factor YY1 represses cell-free replication from human papillomavirus origins

We have established cell-free replication for the human papillomavirus type 18 (HPV-18) origin of replication (ori)-containing DNA by using purified HPV-18 E1 and E2 gene products expressed as fusion proteins in Escherichia coli. The transcription factor YY1 has been shown to regulate RNA transcription by binding to a sequence overlapping the putative E1 protein binding site in the HPV-18 ori. We show that exogenously added YY1 fusion protein inhibited HPV-18 ori replication. Cotransfection of YY1 expression vectors also inhibited transient replication in 293 cells. However, inhibition did not appear to be mediated by binding to its cognate site in the ori as YY1 also inhibited the replication of the HPV-11 ori, which does not have a known or suspected YY1 binding site. Moreover, inhibition was not alleviated by the inclusion of YY1 binding oligonucleotides in the replication reaction mixtures. Rather, we demonstrated a direct interaction between purified fusion E2 protein and fusion YY1 protein by the pull-down assay and a partial restoration of replication activity by an elevated E2 protein concentration. These results suggest that YY1 can inhibit HPV ori replication by interfering with E2 protein functions.