#RecA protein

Repair Pathway

RecA activates SOS response by inducing cleavage of

RecA forms helical nucleoprotein filament on single-stranded DNA generated at lesion-stalled forks, with ATP stabilizing active extended conformation termed RecA*. Activated filament functions as coprotease enhancing intrinsic autoproteolytic activity of LexA, UmuD, and phage lambda repressor. LexA self-cleavage occurs between alanine 84 and glycine 85 within flexible linker connecting N-terminal DNA-binding domain and C-terminal dimerization domain. Cleavage splits repressor, rendering fragments unable to dimerize and bind SOS box. Consequently operons derepress without RecA directly cutting

Ref: PNAS 2021 Cryo-EM RecA-LexA filament structure autocleavage; Alberts Ch.5 RecA coprotease mechanism SOS activation

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Repair Pathway

SOS response is regulated by

SOS response encompasses global induction of more than forty DNA repair and tolerance genes triggered by extensive single-stranded DNA at stalled replication forks. Central negative regulator is LexA repressor, homodimer binding SOS boxes with consensus CTGTN8ACAG in promoter regions, repressing transcription including own lexA gene and recA, uvr, umuDC operons. Under steady growth, LexA maintains low expression. After DNA damage, activated RecA nucleofilament stimulates LexA autocleavage at Ala84-Gly85, causing dissociation from operators and derepression. Temporal induction occurs as LexA af

Ref: Alberts Molecular Biology of Cell, 7th ed., Chapter 5: SOS regulon LexA repressor control; Scielo 2020 SOS regulation E coli

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Recombination

RecA protein primarily binds to

RecA family recombinases exhibit strong preference for single-stranded DNA as assembly platform, even though they can weakly associate with duplex. ATP-bound RecA binds cooperatively to ssDNA generated by RecBCD or MRX resection, forming extended nucleoprotein filament where DNA is stretched 1.5-fold and organized in triplets for homology sampling. This filament discriminates against dsDNA because secondary structure melting and exposure of bases is required. In vivo, mediator proteins assist replacement of SSB or RPA on ssDNA to facilitate filament nucleation and subsequent homology search an

Ref: Watson et al., Molecular Biology of the Gene, 7th ed., Chapter 10: RecA Binding to ssDNA Filament Formation

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Recombination

Strand invasion during homologous recombination is mediated by

Resected 3' single-stranded overhangs are rapidly coated by RPA, which must be replaced by recombinase to become invasive. In bacteria RecA, and in eukaryotes Rad51 and meiosis-specific Dmc1, polymerize directionally on ssDNA into right-handed helical nucleoprotein filaments with ATP. This filament stretches DNA and performs homology search via base-triplet sampling, then catalyzes pairing and strand exchange with intact duplex donor. Invasion creates a displacement loop and provides 3'-OH primer for repair synthesis, establishing heteroduplex DNA without requiring ATP hydrolysis for pairing i

Ref: NCBI Bookshelf, Molecular Cell Biology, Homologous Recombination – RecA/Rad51 Mediated Strand Invasion

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