THE CHI-PSI SUBUNITS OF DNA-POLYMERASE-III HOLOENZYME BIND TO SINGLE-STRANDED DNA-BINDING PROTEIN (SSB) AND FACILITATE REPLICATION OF AN SSB-COATED TEMPLATE

A complex of the chi and psi proteins is required to confer resistance to high levels of glutamate on the DNA polymerase III holoenzyme-cata lyzed reaction (Olson, M., Dallmann, H. G., and McHenry, C. (1995) J. Biol Chem. 270, 29570-29577), We demonstrate that this salt resistance also requires templates to be coated with the Escherichia coli single -stranded DNA-binding protein (SSB). We show that this is the result o f a direct chi psi-SSB interaction that is strengthened approximately 1000-fold when SSB is bound to DNA. On model oligonucleotide templates , DNA polymerase III core is inhibited by SSB. We show that the minima l polymerase assembly that will synthesize DNA on SSB-coated templates is polymerase III-tau-psi chi. gamma, the alternative product of the dnaX gene, will not replace tau in this reaction, indicating that tau' s unique ability to bind to DNA polymerase III holding chi psi in the same complex is essential. All of our findings are consistent with chi psi strengthening DNA polymerase III holoenzyme interactions with the SSB-coated lagging strand at the replication fork, facilitating compl ex assembly and elongation.