τ binds and organizes Escherichia coli replication proteins through distinct domains -: Domain III, shared by γ and τ binds δδ′ and xψ

The DnaX complex of the DNA polymerase holoenzyme assembles the beta (2) processivity factor onto the primed template enabling highly processive replication. The key ATPases within this complex are tau and gamma, alternative frameshift products of the dnaX gene. Of the five domains of tau, I-III are shared with gamma In vivo, gamma binds the auxiliary subunits delta delta' and chi psi (Glover, B. P., and McHenry, C. S. (2000) J. Biol. Chem. 275, 3017-3020), To localize delta delta' and chi psi binding domains within gamma domains I-III, we measured the binding of purified biotin-tagged DnaX proteins lacking specific domains to delta delta' and chi psi by surface plasmon resonance. Fusion proteins containing either DnaX domains I-III or domains III-V bound delta delta' and chi psi subunits, A DnaX protein only containing domains I and II did not bind delta delta' or chi psi The binding affinity of chi psi for DnaX domains I-III and domains III-V was the same as that of chi psi for full-length tau, indicating that domain III contained all structural elements required for chi psi binding. Domain III of tau also contained delta delta' binding sites, although the interaction between delta delta' and domains III-V of tau was 10-fold weaker than the interaction between SS' and full length tau. The presence of both S and chi psi strengthened the delta'-C(0)tau interaction by at least 15-fold. Domain III was the only domain common to all of tau fusion proteins whose interaction with delta' was enhanced in the presence of S and chi psi. Thus, domain III of the DnaX proteins not only contains the delta delta' and chi psi binding sites but also contains the elements required for the positive cooperative assembly of the DnaX complex.