Locking down SOS Mutagenesis Repression in a Dynamic Pathogen.

The article "The DdrR coregulator of the Acinetobacter baumannii mutagenic DNA damage response potentiates UmuDAb repression of error-prone polymerases" in this issue of the J Bacteriol, (D. Cook, M. D. Flannigan, B. V. Candra, K. D. Compton, and J. M. Hare., J Bacteriol 204:e00165-22, 2022, https://doi.org/10.1128/jb.00165-22) reveals a more detailed understanding of the regulatory mechanism of the SOS response in Acinetobacter baumannii. This information provides novel targets for development of antimicrobial therapies against this ESKAPE pathogen and new insight into the complex regulation of the SOS stress-response.

Sensitivity of an Acinetobacter baylyi mpl mutant to DNA damage.

A mutant derived from Acinetobacter baylyi ADP1 was isolated from a screen for genes involved in the response to DNA damage. This derivative has an insertion in the mpl gene which encodes a peptidoglycan-recycling protein. We demonstrate that the insertion renders cells sensitive to mitomycin C and to UV.

A constitutively expressed, truncated umuDC operon regulates the recA-dependent DNA damage induction of a gene in Acinetobacter baylyi strain ADP1.

In response to environmentally caused DNA damage, SOS genes are up-regulated due to RecA-mediated relief of LexA repression. In Escherichia coli, the SOS umuDC operon is required for DNA damage checkpoint functions and for replicating damaged DNA in the error-prone process called SOS mutagenesis. In the model soil bacterium Acinetobacter baylyi strain ADP1, however, the content, regulation, and function of the umuDC operon are unusual. The umuC gene is incomplete, and a remnant of an ISEhe3-like transposase has replaced the middle 57% of the umuC coding region. The umuD open reading frame is intact, but it is 1.5 times the size of other umuD genes and has an extra 5' region that lacks homology to known umuD genes. Analysis of a umuD::lacZ fusion showed that umuD was expressed at very high levels in both the absence and presence of mitomycin C and that this expression was not affected in a recA-deficient background. The umuD mutation did not affect the growth rate or survival after UV-induced DNA damage. However, the UmuD-like protein found in ADP1 (UmuDAb) was required for induction of an adjacent DNA damage-inducible gene, ddrR. The umuD mutation specifically reduced the DNA damage induction of the RecA-dependent DNA damage-inducible ddrR locus by 83% (from 12.9-fold to 2.3-fold induction), but it did not affect the 33.9-fold induction of benA, an unrelated benzoate degradation gene. These data suggest that the response of the ADP1 umuDC operon to DNA damage is unusual and that UmuDAb specifically regulates the expression of at least one DNA damage-inducible gene.

The expression of the Acinetobacter calcoaceticus recA gene increases in response to DNA damage independently of RecA and of development of competence for natural transformation.

Using the lacZ operon fusion technique, the transcriptional control of the Acinetobacter calcoaceticus recA gene was studied. A low (approximately twofold) inductive capacity was observed for compounds that damage DNA and/or inhibit DNA replication, e.g. methyl methanesulfonate, mitomycin C, UV light and nalidixic acid. Induction of the recA gene by DNA damage was independent of functional RecA. The presence of the recA promoter region on a multicopy plasmid had the same effect on recA transcription as the presence of DNA-damaging agents. Thus, recA expression in A. calcoaceticus appears to be regulated in a novel fashion, possibly involving a non-LexA-like repressor. Regulation of the recA gene in A. calcoaceticus appears not to be part of a regulon responsible for competence for natural transformation: in cells exhibiting extremely low transformation frequencies, the level of transcription of the recA gene was found to be comparable to the level found in cells in the state of maximal competence.