| The LexA protein is the transcriptional repressor of the SOS system, a tightly regulated network in response to DNA damage and repair (Witkin, 1976; Walker, 1984). LexA is widespread across the Bacteria Domain and exhibits considerable variation in its amino acid composition and its preferred binding sites in DNA. Under normal conditions, LexA represses the transcription of many DNA repair proteins by binding to SOS "boxes" located in their promoter regions. After extensive DNA damage, repression is relieved when LexA, which contains a latent protease domain, cleaves itself. Autoproteolysis is triggered when LexA binds to accumulating RecA-ssDNA-ATP repair filaments. To address our first question, 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 motif. The wings of these two DNA binding domains dock into the same minor groove of the DNA. Their synergistic contacts may explain the invariance of spacing between the two half-sites of the E. coli SOS boxes. Furthermore, a series of binding assays provided additional information regarding LexA's sequence preference toward the spacing region between the two half-sites. To address how LexA adapts to a thermophilic environment, we analyzed LexA from two different species. We solved a crystal structure of LexA from Thermotoga maritima for comparison with the E. coli protein structure. Comparisons of both LexA structures reveal several differences within the amino acid compositions and secondary structures. These observations from the analysis of the LexA-DNA complex and T. maritima and E. coli LexA proteins provide a general structural framework for LexA-DNA recognition and association and a glimpse into the evolutionary relationships of protein structure, stability, and thermophilic adaptation. |
