Devoted to the lagging strand -: the χ subunit of DNA polymerase III holoenzyme contacts SSB to promote processive elongation and sliding clamp assembly

Escherichia coli DNA polymerase III holoenzyme contains 10 different subunits which assort into three functional components: a core catalytic unit containing DNA polymerase activity, the beta sliding clamp that encircles DNA for processive replication, and a multi-subunit clamp loader apparatus called gamma complex that uses ATP to assemble the beta clamp onto DNA. We examine here the function of the chi subunit of the gamma complex clamp loader. Omission of chi from the holoenzyme prevents contact with single-stranded DNA-binding protein (SSB) and lowers the efficiency of clamp loading and chain elongation under conditions of elevated salt. We also show that the product of a classic point mutant of SSB, SSB-113, lacks strong affinity for chi and is defective in promoting clamp loading and processive replication at elevated ionic strength, SSB-113 carries a single amino acid replacement at the penultimate residue of the C-terminus, indicating the C-terminus as a site of interaction with chi. Indeed, a peptide of the 15 C-terminal residues of SSB is sufficient to bind to chi. These results establish a role for the chi subunit in contacting SSB, thus enhancing the clamp loading and processivity of synthesis of the holoenzyme, presumably by helping to localize the holoenzyme to sites of SSB-coated ssDNA.