The DdrR Coregulator of the Acinetobacter Baumannii Mutagenic DNA Damage Response Potentiates UmuDAb Repression of Error-Prone Polymerases

Acinetobacter baumannii strain 17978 is an opportunistic pathogen with a unique DNA damage repair response that lacks the LexA repressor but induces ~150 genes after DNA damage. It uses the UmuD homolog UmuDAb and the small protein DdrR, unique to Acinetobacter, to repress multiple horizontally acquired error-prone polymerase genes through an unknown mechanism. We used reverse transcription-quantitative PCR and immunoblotting to elucidate UmuDAb regulatory requirements and DdrR contributions to the corepression of this specialized regulon. Mutations in the putative UmuDAb helix-turn-helix (HTH) domain could not repress the expression of the UmuDAb/DdrR regulon. A insertion mutation in these HTH mutant backgrounds produced even greater derepression of the regulon, suggesting that DdrR exerts an additional level of control over this mutagenic response. These HTH mutant A. baumannii cells overexpressed UmuDAb, cleaving it after treatment with the DNA-damaging agent mitomycin C. This showed that DdrR was not required for UmuDAb self-cleavage and that UmuDAb repression and self-cleavage actions were independent. An uncleavable mutant with an A-to-Y change at position 83 (A83Y) could neither induce the UmuDAb/DdrR regulon nor conduct SOS mutagenesis. However, a prophage-encoded operon was still partially induced after DNA damage in this mutant. Surprisingly, that prophage's putative repressor was dispensable for prophage-encoded induction, implying another repressor's involvement. This study revealed that UmuDAb behaves like LexA, requiring an N-terminal HTH motif for repression and C-terminal self-cleavage for gene induction and subsequent SOS mutagenesis, and DdrR cooperates with it to exert an additional level of repressive control on this pathogen's mutagenic response to DNA damage. Acinetobacter baumannii is a nosocomial pathogen that acquires antibiotic resistance genes through conjugative transfer and carries out a robust mutagenic DNA damage response. After exposure to conditions typically encountered in health care settings, such as antibiotics, UV light, and desiccation, this species induces error-prone UmuD'C polymerases. This mutagenic capability increases A. baumannii survival and virulence and is regulated by the UmuDAb/DdrR corepressor system unique to the Acinetobacter genus. Our study has revealed that the DdrR protein provides an additional layer of control in preventing mutagenic polymerase expression by enhancing UmuDAb repression actions. Understanding these repressors could lead to new drug targets, as multidrug resistance in hospital-acquired infections has decreased treatment options, with limited new drugs being developed.