Effect of a Defective Clamp Loader Complex of DNA Polymerase III on Growth and SOS Response in Pseudomonas aeruginosa .

DNA polymerase III (Pol III) is the replicative enzyme in bacteria. It consists of three subcomplexes, the catalytic core, the β clamp, and the clamp loader. While this complex has been thoroughly characterized in the model organism Escherichia coli , much less is known about its functioning and/or its specific properties in other bacteria. Biochemical studies highlighted specific features in the clamp loader subunit ψ of Pseudomonas aeruginosa as compared to its E. coli counterpart, and transposon mutagenesis projects identified the ψ-encoding gene holD among the strictly essential core genes of P . aeruginosa . By generating a P . aeruginosa holD conditional mutant, here we demonstrate that, as previously observed for E . coli holD mutants, HolD-depleted P . aeruginosa cells show strongly decreased growth, induction of the SOS response, and emergence of suppressor mutants at high frequency. However, differently from what was observed in E. coli , the growth of P . aeruginosa cells lacking HolD cannot be rescued by the deletion of genes for specialized DNA polymerases. We also observed that the residual growth of HolD-depleted cells is strictly dependent on homologous recombination functions, suggesting that recombination-mediated rescue of stalled replication forks is crucial to support replication by a ψ-deficient Pol III enzyme in P . aeruginosa .