RecA

RecA is a recombinase enzyme essential for homologous recombination and DNA repair in bacteria. The recA gene encodes a 37–40 kDa protein that forms helical nucleoprotein filaments on single-stranded DNA and orchestrates strand exchange during recombination. RecA is functionally related to eukaryotic Rad51 and Dmc1.

Once bound to ssDNA, RecA catalyzes homology search and strand invasion in an ATP-dependent reaction. The RecA-ssDNA filament pairs the damaged ssDNA with a homologous duplex, promoting strand exchange to form a displacement loop and eventually Holliday junctions. The filament also facilitates annealing and DNA synthesis during repair and replication restart. SSB proteins initially coat ssDNA and are displaced by RecA with the help of loading factors.

RecA activity is tightly regulated. The RecA-ssDNA filament stimulates LexA autoproteolysis, triggering the SOS response and induction of DNA damage tolerance genes, including error-prone polymerases. In Escherichia coli and many bacteria, RecA loading onto DNA ends or gaps is assisted by RecBCD (double-strand break ends) or RecFOR (single-stranded gaps) pathways, respectively, with additional accessory proteins like RecF, RecO, and RecR modulating efficiency.

RecA is widespread in bacteria; eukaryotic homologs Rad51 and Dmc1 perform similar recombination roles, while archaea have RadA-related proteins. RecA's activity is central to genome maintenance and the cellular response to DNA damage.