Magnesium ions enhance the transfer of human papillomavirus E2 protein from non-specific to specific binding sites

The human papillomavirus 16 E2 protein binds to four specific DNA sequences present within the HPV 16 genome and regulates viral gene expression and DNA replication. However, the E2 protein can also bind tightly to non-specific DNA sequences. Here, we show that in binding reactions which contain an excess of non-specific DNA, magnesium ions enhance the binding of E2 to its specific sites. In contrast, in the absence of non-specific DNA, magnesium ions have no effect on the binding of E2 to specific sites. Although these data suggest that magnesium ions decrease the binding of E2 to non-specific DNA, gel retardation assays show that these ions have no effect on the binding of E2 to short non-specific DNA fragments and have only a minor effect on the binding of E2 to long non-specific DNA fragments. We also show that the binding of E2 to long fragments of non-specific DNA is highly cooperative. The E2-non-specific DNA complexes formed in the absence of magnesium ions are highly stable. However, the addition of specific DNA to E2-non-specific DNA complexes formed in the presence of magnesium ions rapidly results in the formation of E2-specific DNA complexes. Our data suggest that magnesium ions facilitate the transfer of E2 from non-specific binding sites to specific binding sites, and help to explain how E2 is able to direct human papillomavirus transcription and DNA replication in intact cells.