The Acinetobacter regulatory UmuDAb protein cleaves in response to DNA damage with chimeric LexA/UmuD characteristics

In the DNA damage response of most bacteria, UmuD forms part of the error-prone (UmuD'(2) )C polymerase V and is activated for this function by self-cleavage after DNA damage. However, the umuD homolog (umuDAb) present throughout the Acinetobacter genus encodes an extra N-terminal region, and in Acinetobacter baylyi, regulates transcription of DNA damage-induced genes. UmuDAb expressed in cells was correspondingly larger (24 kDa) than the Escherichia coli UmuD (15 kDa). DNA damage from mitomycin C or UV exposure caused UmuDAb cleavage in both E. coli wild-type and ΔumuD cells on a timescale resembling UmuD, but did not require UmuD. Like the self-cleaving serine proteases LexA and UmuD, UmuDAb required RecA for cleavage. This cleavage produced a UmuDAb' fragment of a size consistent with the predicted cleavage site of Ala83-Gly84. Site-directed mutations at Ala83 abolished cleavage, as did mutations at either the Ser119 or Lys156 predicted enzymatic residues. Co-expression of the cleavage site mutant and an enzymatic mutant did not allow cleavage, demonstrating a strictly intramolecular mechanism of cleavage that more closely resembles the LexA-type repressors than UmuD. These data show that UmuDAb undergoes a post-translational, LexA-like cleavage event after DNA damage, possibly to achieve its regulatory action.