title: Chaperoning of a replicative polymerase onto a newly assembled DNA-bound sliding clamp by the clamp loader.
abstract: Cellular replicases contain multiprotein ATPases that load sliding clamp processivity factors onto DNA. We reveal an additional role for the DnaX clamp loader: chaperoning of the replicative polymerase onto a clamp newly bound to DNA. We show that chaperoning confers distinct advantages, including marked acceleration of initiation complex formation. We reveal a requirement for the tau form of DnaX complex to relieve inhibition by single-stranded DNA binding protein during initiation complex formation. We propose that, after loading beta(2), DnaX complex preserves an SSB-free segment of DNA immediately downstream of the primer terminus and chaperones Pol III into that position, preventing competition by SSB. The C-terminal tail of SSB stimulates reactions catalyzed by tau-containing DnaX complexes through a contact distinct from the contact involving the chi subunit. Chaperoning of Pol III by the DnaX complex provides a molecular explanation for how initiation complexes form when supported by the nonhydrolyzed analog ATPgammaS.
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Downey
authors.forename: Charles S
authors.initials: CS
authors.lastname: McHenry
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D004247:DNA / Q000235:genetics / Q000378:metabolism*; D004258:DNA Polymerase III / Q000235:genetics / Q000378:metabolism*; D004261:DNA Replication; D004268:DNA-Binding Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology*; D018832:Molecular Chaperones / Q000378:metabolism*; D011485:Protein Binding; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; C103076:DnaX protein, Bacteria; D018832:Molecular Chaperones; D021122:Protein Subunits; D000255:Adenosine Triphosphate; D004247:DNA; D004258:DNA Polymerase III