Competition for DNA Binding Sites between the Short and Long Forms of E2 Dimers Underlies Repression in Bovine Papillomavirus Type 1 DNA Replication Control

Abstract

Papillomaviruses establish a long-term latency in vivo by maintaining their genomes as nuclear plasmids in proliferating cells. Bovine papillomavirus type 1 encodes two proteins required for viral DNA replication: the helicase E1 and the positive regulator E2. The homodimeric E2 is known to cooperatively bind to DNA with E1 to form a preinitiation complex at the origin of DNA replication. The virus also codes for two short forms of E2 that can repress viral functions when overexpressed, and at least one copy of the repressor is required for stable plasmid maintenance in transformed cells. Employing a tetracycline-regulated system to control E1 and E2 production from integrated loci, we show that the short form of E2 negatively regulates DNA replication. We also found that the short form could repress replication in a cell-free replication system and that the repression requires the DNA binding domain of the protein. In contrast, heterodimers of the short and long forms were activators and, by footprint analysis, were shown to be as potent as homodimeric E2 in loading E1 to its cognate site. DNA binding studies show that when E1 levels are low and are dependent upon E2 for occupancy of the origin site, the repressor can block E1-DNA interactions. We conclude that DNA replication modulation results from competition between the different forms of E2 for DNA binding. Given that heterodimers are active and that the repressor form of E2 shows little cooperativity with E1 for DNA binding, this protein is a weak repressor.