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.

Ultraviolet B (UVB) exposure causes damage to skin and represents the primary etiological agent for skin cancer formation. UVB induces DNA damage and apoptosis in epidermal cells. In this study, we demonstrated that UVB activated autophagy in JB6 epidermal cells, which was evident by the formation of LC3 puncta, the induction of LC3 lipidation, the increase in beclin 1 expression, and the decrease in the levels of p62. Autophagy appeared to be a protective response to UVB-induced damage because inhibition of autophagy exacerbated UVB-induced cell death, and stimulation of autophagy offered protection. Furthermore, we demonstrated that glycogen synthase kinase 3β (GSK3β) was involved in UVB-induced autophagy. UVB inhibited GSK3β activation by simultaneously enhancing phosphorylation at Ser9 and suppressing Tyr216 phosphorylation. GSK3β negatively regulated autophagy; overexpression of wild-type or S9A (constitutive-active) GSK3β mutant inhibited UVB-mediated autophagy, while overexpression of a dominant-negative K85R mutant enhanced UVB-mediated autophagy. Inhibition of GSK3β also offered protection against UVB-mediated damage. UVB activated AMP-activated protein kinase (AMPK), an important regulator of autophagy through the inhibition of GSK3β. Taken together, our results suggest that UVB-stimulated autophagy is a protective response for epidermal cells and is mediated by the GSK3β/AMPK pathway.