UmuDAb: An Error-Prone Polymerase Accessory Homolog Whose N-Terminal Domain Is Required for Repression of DNA Damage Inducible Gene Expression in <i>Acinetobacter baylyi</i>

<div><p>In many bacteria, the DNA damage response induces genes (SOS genes) that were repressed by LexA. LexA represses transcription by binding to SOS promoters via a helix-turn-helix motif in its N-terminal domain (NTD). Upon DNA damage, LexA cleaves itself and allows induction of transcription. In <i>Acinetobacter baumannii</i> and <i>Acinetobacter baylyi</i>, multiple genes are induced by DNA damage, and although the <i>Acinetobacter</i> genus lacks LexA, a homolog of the error-prone polymerase subunit UmuD, called UmuDAb, regulates some DNA damage-induced genes. The mechanism of UmuDAb regulation has not been determined. We constructed UmuDAb mutant strains of <i>A</i>. <i>baylyi</i> to test whether UmuDAb mediates gene regulation through LexA-like repressor actions consisting of relief of repression through self-cleavage after DNA damage. Real-time quantitative PCR experiments in both a null <i>umuDAb</i> mutant and an NTD mutant showed that the DNA damage-inducible, UmuDAb-regulated gene <i>ddrR</i> was highly expressed even in the absence of DNA damage. Protein modeling identified a potential LexA-like helix-turn-helix structure in the UmuDAb NTD, which when disrupted, also relieved <i>ddrR</i> and <i>umuDAb</i> repression under non-inducing conditions. Mutations in a putative SOS box in the shared <i>umuDAb-ddrR</i> promoter region similarly relieved these genes’ repression under non-inducing conditions. Conversely, cells possessing a cleavage-deficient UmuDAb were unable to induce gene expression after MMC-mediated DNA damage. This evidence of a UmuDAb repressor mechanism was contrasted with the failure of <i>umuDAb</i> to complement an <i>Escherichia coli umuD</i> mutant for UmuD error-prone DNA replication activity. Similarly, <i>A</i>. <i>baumannii</i> null <i>umuDAb</i> mutant cells did not have a reduced UmuDˊ₂UmuC-mediated mutation rate after DNA damage, suggesting that although this UmuDAb protein may have evolved from a <i>umuDC</i> operon in this genus, it now performs a LexA-like repressor function for a sub-set of DNA damage-induced genes.</p>