Visualization of the Annealing of Complementary Single-stranded DNA Catalyzed by the Herpes Simplex Virus Type 1 ICP8 SSB/Recombinase

The rate of annealing of long linear complementary single-stranded (ss) DNAs can be increased greatly by certain DNA-binding proteins including the herpes simplex virus type 1 ICP8 SSB/recombinase. Using electron microscopy, we have investigated the DNA–protein structures involved in ICP8-mediated DNA annealing. We show that the formation of superhelical ICP8–ssDNA filaments is required for annealing. Two superhelices interact with each other to form a coiled-coil, which is the intermediate in annealing. In this process, the superhelices likely rotate and translocate relative to each other. Psoralen/UV photocrosslinking studies revealed that meta-stable contacts form at sites of limited sequence homology during the annealing. Partial proteolysis of ICP8 in the protein–ssDNA complexes showed that Mg2+ induces conformational changes in the N-terminal region (amino acid residues 1–305) of ICP8. In addition to Mg2+, Ca2+ and, to a significantly lesser extent, Cu2+ and Mn2+, were found to induce superhelix formation of the ICP8–ssDNA filament and to facilitate annealing. Mechanisms for how the coiled-coil structures facilitate annealing are discussed.