The initiator protein E1 binds to the bovine papillomavirus origin of replication as a trimeric ring-like structure

The replication initiator protein El binds to the origin of replication of bovine papillomavirus in several forms. El can bind to its recognition sequence as a monomer together with the viral transcription factor E2, or as a trimeric El complex. The trimerization of El is mediated by the sequence-specific binding of El to DNA, and results in an El complex that is linked topologically to the DNA because the three molecules of El form a ring-like structure that encircles the DNA. These results demonstrate that El utilizes unusual mechanisms for sequence-specific binding to DNA and for the generation of a structure that encircles the DNA. We believe that these forms of El bound to the origin of replication represent intermediates in a transition in the function of El, from a sequence-specific origin of replication recognition protein to a form of El that is competent for the initiation of viral DNA replication.