THE E1-PROTEIN OF BOVINE PAPILLOMA VIRUS-1 IS AN ATP-DEPENDENT DNA HELICASE

For efficient DNA replication of papilloma-viruses, only two viral-enc oded proteins, E1 and E2, are required. Other proteins and factors are provided by the host cell. E2 is an enhancer of both transcription an d replication and is known to help E1 bind cooperatively to the origin of DNA replication. E1 is sufficient for replication in extracts prep ared from permissive cells, but the activity is enhanced by E2. Here w e show that purified E1 can act as an ATP-dependent DNA helicase. To m easure this activity, we have used strand displacement, unwinding of t opologically constrained DNA, denaturation of duplex fragments, and el ectron microscopy. The ability of E1 to unwind circular DNA is found t o be independent of origin-specific viral DNA sequences under a variet y of experimental conditions. In unfractionated cellular extracts, E1- dependent viral DNA replication is origin-dependent, but at elevated E 1 concentrations, replication can occur on non-origin-containing DNA t emplates. This conversion from an origin-dependent replication system to a nonspecific initiator system is discussed in the context of the c urrent understanding of the initiation of chromosomal DNA replication.