Homodimerization and Heterodimerization Requirements of Acinetobacter baumannii SOS Response Coregulators UmuDAb and DdrR revealed by Two-Hybrid Analyses.

The multi-drug resistant pathogen Acinetobacter baumannii displays unusual control of its SOS mutagenesis genes, as it does not encode a LexA repressor, but instead employs the UmuDAb repressor and a small DdrR protein that is uniquely found in Acinetobacter species. We used bacterial adenylate cyclase two-hybrid analyses to determine if UmuDAb and DdrR coregulation might involve physical interactions. Neither quantitative nor qualitative assays showed UmuDAb interaction with DdrR. DdrR hybrid proteins, however, demonstrated modest head-to-tail interactions in a qualitative assay. The similarity of UmuDAb to the homodimer-forming polymerase manager UmuD and LexA repressor proteins suggested that it may form dimers, which we observed. UmuDAb homodimerization required a free C-terminus, and either small truncations or addition of a histidine tag at the C-terminus abolished this homodimerization. Amino acid N100, crucial for UmuD dimer formation, was dispensable if both C-termini were free to interact. However, mutation of G124, necessary for LexA dimerization, yielded significantly less UmuDAb dimerization, even if both C-termini were free. This suggests that UmuDAb forms dimers like LexA, but may not co-regulate gene expression involving a physical association with DdrR. The homodimerization of these coregulators provides insight into a LexA-independent, coregulatory process of controlling a conserved bacterial action such as the mutagenic DNA damage response.