Structure of the SSB-DNA polymerase III interface and its role in DNA replication

Interactions between single-stranded DNA-binding proteins (SSBs) and the DNA replication machinery are found in all organisms, but the roles of these contacts remain poorly defined. In Escherichia coli, SSB's association with the chi subunit of the DNA polymerase III holoenzyme has been proposed to confer stability to the replisome and to aid delivery of primers to the lagging-strand DNA polymerase. Here, the SSB-binding site on chi is identified crystallographically and biochemical and cellular studies delineate the consequences of destabilizing the chi/SSB interface. An essential role for the chi/SSB interaction in lagging-strand primer utilization is not supported. However, sequence changes in v that block complex formation with SSB lead to salt-dependent uncoupling of leading- and lagging-strand DNA synthesis and to a surprising obstruction of the leading-strand DNA polymerase in vitro, pointing to roles for the chi/SSB complex in replisome establishment and maintenance. Destabilization of the chi/SSB complex in vivo produces cells with temperature-dependent cell cycle defects that appear to arise from replisome instability.