The Escherichia coli SOS system (1) includes 40 genes that are under the control of the LexA repressor (2). During normal cellular growth, LexA binds to the operator of the repair genes as a dimer (3). When DNA damage occurs, RecA changes into an active (ssDNA- and ATP-bound) state and interacts with LexA. This interaction stimulates LexA into the active conformation for autocleavage. The chemistry follows a Ser-Lys dyad mechanism, which occurs within the C-terminal dimerization domain. Cleavage inactivates LexA, therefore, derepresses the SOS system. Previous study has been focused on EcLexA. In this study, we report a 1.39Å resolution crystal structure of a Thermotoga maritima LexA mutant. This mutant, K156A, is a non-cleavable mutant that binds tightly to RecA. TmLexA shares 44% identity at the amino acid sequence level with EcLexA. Structural comparisons between the two proteins note differences that may contribute to the enhanced thermostability of TmLexA.

References and Footnotes

1. Walker, G. C. Annu Rev Biochem 54, 425 (1985).
2. Fernandez de Henestrosa A. R., Ogi, T., Aoyagi, S., Chafin, D., Hayes, J. J., Ohmorri, H., Woodgate, R. Mol Microbiol 35, 1560 (2000).
3. Kim, B. and Little, J. W. Science 255, 203 (1992).