FIG. 9.

“Handover” model for assembly of hexamers at the replication origin. Monomers of large T antigen (spheres) bind cooperatively to the four binding sites in origin DNA to form a complex as shown in Fig. 9. In the presence of ATP, bound monomers in contact with each other dissociate from the G(A/G)GGC sequences to which they are bound and rearrange to form a hexamer at the center of site 1/2, which can unwind DNA and allow initiation of DNA replication. A second hexamer may subsequently form by a similar rearrangement.