DdrR acts as a DNA damage-induced cell division inhibitor in Acinetobacter baumannii

The genus Acinetobacter possesses non-canonical genes and responses to DNA damage. It encodes no LexA homolog to repress the ~150 SOS genes induced after exposure to various DNA-damaging agents, but two SOS regulators, UmuDAb and DdrR, repress both themselves and error-prone polymerases that generate mutations after DNA damage. Further differences include the lack of the Gram-negative cell division inhibitor (CDI), SulA, in members of the Moraxellaceae family to which Acinetobacter belongs. However, ddrR, which lacks homologs outside the Acinetobacter genus, is transcribed divergently from umuDAb, like small CDIs of Gram-positive species lacking SulA, leading us to explore the possibility that DdrR inhibited cell division after DNA damage in the opportunistic Acinetobacter baumannii. Like lexA mutants of Escherichia coli and Caulobacter crescentus that overexpress CDIs SulA and SidA, respectively, a null umuDAb mutant that overexpresses ddrR formed filaments in the absence of DNA damage and further increased its cell length after DNA damage. A strain expressing a non-cleavable UmuDAb showed neither of these phenotypes, implicating a UmuDAb-repressed gene in the inhibition of cell division. However, ΔumuDAb ΔddrR double mutant cells regained a wild-type cell length and did not form filaments after DNA damage, and ddrR mutant cells had an impaired filamentation response to DNA damage relative to wild type. Conversely, wild-type cells overexpressing ddrR became filamented in the absence of DNA damage. umuDAb mutant cells overexpressing ddrR exhibited impaired growth as measured by increased doubling times, lower optical density, and reduced cell viability. The rescue of these growth impairments by ddrR mutation suggested that DdrR functions as a CDI in the DNA damage response of Acinetobacter.IMPORTANCEIn hospital environments, the multi-drug-resistant opportunistic pathogen Acinetobacter baumannii incurs DNA damage from antibiotics, UV exposure, and desiccation, which induces multiple error-prone polymerases that can cause clinically relevant antibiotic resistance. However, while Acinetobacter species lack the common DNA damage response proteins LexA and SulA, they also show the typical DNA damage response of a cell division checkpoint. We found that the small SOS protein DdrR inhibited cell division like Gram-positive cell division inhibitor (CDI) proteins. After DNA damage, DdrR was required for the cell filamentation response, while increased DdrR expression impaired cell growth and viability. Uncovering a novel mechanism of controlling growth inhibition after DNA damage presents an opportunity to define a new model of inhibiting cell division for non-Enterobacteriaceae Gram-negative Gammaproteobacteria.