Transcription factor-dependent loading of the E1 initiator reveals modular assembly of the papillomavirus origin melting complex

Replication of bovine papillomavirus type 1 DNA absolutely requires the viral transcription factor E2 as well as the initiator Fl, although Fl alone has all the activities expected of an initiator protein. El assembles on the DNA in a stepwise fashion and undergoes a transition in activities from site-specific DNA-binding protein to mobile helicase. Complex assembly is assisted by the viral transcription factor E2 at two levels. E2 acts generally as a specificity factor, which through cooperative binding with El generates an initial Fl complex containing three Fl dimers bound to ori on one face of the DNA, El-ori. Furthermore, E2 can promote the transition to an ori melting complex by recruiting additional Fl molecules to ori, effectively reducing the Fl concentration required for ori melting. This reaction is dependent on an Ea-binding site positioned distal to the precursor E1-ori complex. The final origin melting complex has two subunits that each encircle the DNA and function independently to melt ori. The assembly pathway we describe has implication for understanding DNA melting and unwinding reactions, which are generally poorly understood.