tau binds and organizes Escherichia coli replication proteins through distinct domains - Domain IV, located within the unique C terminus of tau, binds the replication fork helicase, Dnab

Interaction between the tau subunit of the DNA polymerase III holoenzyme an d the DnaB helicase is critical for coupling the replicase and the primosom al apparatus at the replication fork (Kim, S., Dallmann, H. G., McHenry, C. S., and Marians, K. J. (1996) Cell 84, 643-650). In the preceding manuscri pt, we reported the identification of five putative structural domains with in the tau subunit (Gao, D., and McHenry, C, (2000) J. Biol, Chem, 275, 443 3-4440). As part of our systematic effort to assign functions to each of th ese domains, we expressed a series of truncated, biotin-tagged tau fusion p roteins and determined their ability to bind DnaB by surface plasmon resona nce on streptavidin-coated surfaces. Only tau fusion proteins containing do main IV bound DnaB, The DnaB-binding region was further limited to a highly basic 66-amino acid residue stretch within domain TV. Unlike the binding o f immobilized tau (4) to the DnaB hexamer, the binding of monomeric domain IV to DnaB(6) was dependent upon the density of immobilized domain IV, indi cating that DnaB(6) is bound by more than one (tau) protomer, This observat ion implies that both the leading and lagging strand polymerases are tether ed to the DnaB helicase via dimeric tau, These double tethers of the leadin g and lagging strand polymerases proceeding through the tau-tau link and an additional tau -DnaB link are likely important for the dynamic activities of the replication fork.