ssb gene duplication restores the viability of ΔholC and ΔholD Escherichia coli mutants.

The HolC-HolD (chi psi) complex is part of the DNA polymerase III holoenzyme (Pol III HE) clamp-loader. Several lines of evidence indicate that both leading-and lagging-strand synthesis are affected in the absence of this complex. The Escherichia coli Delta holD mutant grows poorly and suppressor mutations that restore growth appear spontaneously. Here we show that duplication of the ssb gene, encoding the single-stranded DNA binding protein (SSB), restores Delta holD mutant growth at all temperatures on both minimal and rich medium. RecFOR-dependent SOS induction, previously shown to occur in the Delta holD mutant, is unaffected by ssb gene duplication, suggesting that lagging-strand synthesis remains perturbed. The C-terminal SSB disordered tail, which interacts with several E. coli repair, recombination and replication proteins, must be intact in both copies of the gene in order to restore normal growth. This suggests that SSB-mediated Delta holD suppression involves interaction with one or more partner proteins. ssb gene duplication also suppresses Delta holC single mutant and Delta holC Delta holD double mutant growth defects, indicating that it bypasses the need for the entire chi psi complex. We propose that doubling the amount of SSB stabilizes HolCD-less Pol III HE DNA binding through interactions between SSB and a replisome component, possibly DnaE. Given that SSB binds DNA in vitro via different binding modes depending on experimental conditions, including SSB protein concentration and SSB interactions with partner proteins, our results support the idea that controlling the balance between SSB binding modes is critical for DNA Pol III HE stability in vivo, with important implications for DNA replication and genome stability.