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.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author