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LexA polypeptides contains a two domains: a DNA-binding domain and a dimerization domain. [3] The dimerization domain binds to other LexA polypeptides to form dumbbell shaped dimers. The DNA-binding domain is a variant form of the helix-turn-helix DNA binding motif, [4] and is usually located at the N-terminus of the protein. [1]
The activated form of RecA interacts with the LexA repressor to facilitate the LexA repressor's self-cleavage from the operator. [7] [8] Once the pool of LexA decreases, repression of the SOS genes goes down according to the level of LexA affinity for the SOS boxes. [7] Operators that bind LexA weakly are the first to be fully expressed.
SOS box is the operator to which the LexA repressor binds to repress the transcription of SOS-induced proteins. SOS boxes are found near the promoter of various genes. [1] LexA binds to an SOS box in the absence of DNA damage. In the presence of DNA damage the binding of LexA is inactivated by the RecA activator.
The prokaryotic SOS system is regulated by two key proteins: LexA and RecA. The LexA homodimer is a transcriptional repressor that binds to operator sequences commonly referred to as SOS boxes. In Escherichia coli it is known that LexA regulates transcription of approximately 48 genes including the lexA and recA genes. [62]
Similarly to GAL4/UAS [8] and LexA/LexAop, [9] the Q-system is a binary expression system that allows to express reporters or effectors (e.g. fluorescent proteins, ion channels, toxins and other genes) in a defined subpopulation of cells with the purpose of visualising these cells or altering their function. In addition, GAL4/UAS, LexA/LexAop ...
RecA is a 38 kilodalton protein essential for the repair and maintenance of DNA in bacteria. [2] Structural and functional homologs to RecA have been found in all kingdoms of life.
Normally RecA* binds LexA (a transcription repressor), activating LexA auto-protease activity, which destroys LexA repressor, allowing production of DNA repair proteins. In lysogenic cells, this response is hijacked, and RecA* stimulates cI autocleavage. This is because cI mimics the structure of LexA at the autocleavage site.
Triggering of this system can and has been used as an early sign of DNA damage. Two genes play a key role in the SOS response: lexA encodes a repressor for all the genes in the system, and recA encodes a protein able to cleave the LexA repressor upon activation by an SOS inducing signal (caused in this case by the presence of a genotoxic compound).