Cooperative interactions between replication origin-bound molecules of herpes simplex virus origin-binding protein are mediated via the amino terminus of the protein.

The virally encoded origin binding protein (OBP) of herpes simplex virus (HSV) is required for viral DNA synthesis. OBP binds at the replication origin to initimultienzyme replication complex (Challberg, M. D., and Kelly, T. J. (1989) Annu Rev. Biochem. 58, 671-717), OBP binds to two sites at the replication origin. The sequence-specific interaction of OBP with each binding site is localized to the major groove, and in both HSV origins the two interaction surfaces are in phase, aligned on the same face of the helix (Hazuda, D. J., Perry, H. C., Naylor, A. M., and McClements, W. L. (1991) J. Biol. Chem. 261, 24621-24625). Using native gel electrophoresis, we now demonstrate that OBP binding to the origin is highly cooperative and that cooperativity requires the putative NH2-terminal leucine zipper. Neither the phase nor orientation of the binding sites affect cooperativity, suggesting that the interaction promotes wrapping of origin DNA around the OBP multimer. A comparison of OBP DNase I footprints with the DNase I footprints of a truncated protein defective in cooperativity demonstrates that the interaction between OBPs bound at sites I and II affects the conformation of the intervening DNA, particularly when the phase or orientation of the two sites is different from wild type. OBP may elicit a unique nucleoprotein structure which facilitates unwinding of the origin and/or assembly of the replication complex. We also demonstrate that OBP can exchange binding sites, forming interduplex complexes. This property may be important for reinitiation of DNA replication.