Structure of the LexA-DNA complex and implications for SOS box measurement

The eubacterial SOS system is a paradigm of cellular DNA damage and repair, and its activation can contribute to antibiotic resistance.1,2,3 Under normal conditions, LexA represses the transcription of many DNA repair proteins by binding to SOS “boxes” in their operators. Under genotoxic stress, accumulating RecA-ssDNA-ATP complexes activate LexA for autocleavage. To address how LexA recognizes its binding sites, we determined three crystal structures of Escherichia coli LexA in complex with SOS boxes. These are the first structures reported of a LexA-DNA complex. The DNA-binding domains of the LexA dimer interact with the DNA in the classical fashion of a winged helix-turn-helix (wHTH) motif. However, the wings of these two DNA binding domains bind to the same minor groove of the DNA. These wing-wing contacts may explain why the spacing between the two half-sites of E. coli SOS boxes is invariant.