Definitions

reca'lescent

RecA

RecA is a 38 kilodalton Escherichia coli protein essential for the repair and maintenance of DNA. RecA has a structural and functional homolog in every species in which it has been seriously sought and serves as an archetype for this class of homologous DNA repair proteins. The homologous protein in Homo sapiens is called RAD51.

RecA has multiple activities, all related to DNA repair. In the bacterial SOS response, it has a co-protease function in the autocatalytic cleavage of the LexA repressor and the λ repressor. Its most studied role is in facilitating DNA recombination for the repair of double strand DNA breaks and the exchange of genetic information through sexual reproduction.

E. coli RecA protein also has a major role in homologous recombination. RecA protein binds strongly and in long clusters to ssDNA to form a nucleoprotein filament. It has more than one DNA binding site thus RecA can hold a single strand and double strand together. This feature makes it possible to catalyze a DNA synapsis reaction between a DNA double helix and a homologous region of single stranded DNA. The reaction initiates the exchange of strands between two recombining DNA double helices. After the synapis event, in the heteroduplex region a process called branch migration begins. In branch migration an unpaired region of one of the single strands displaces a paired region of the other single strand, moving the branch point without changing the total number of base pairs. Spontaneous branch migration can occur, however as it generally proceeds equally in both directions it is unlikely to complete recombination efficiently. The RecA protein catalyzes unidirectional branch migration and by doing so makes it possible to complete recombination, producing a region of heteroduplex DNA that is thousands of base pairs long.

RecA protein is a DNA-dependent ATPase, it contains an additional site for binding and hydrolyzing ATP. RecA associates more tightly with DNA when it has ATP bound than when it has ADP bound.

Escherichia coli strains deficient in RecA are useful for cloning procedures in molecular biology laboratories. E. coli strains are often genetically modified to contain a mutant recA locus to ensure the stability of exogenous plasmids: modular circular dsDNA which bacteria replicate with their genome during normal cell growth. Plasmid DNA is taken up by the bacteria under a variety of conditions. Bacteria containing exogenous plasmids are called "transformants". Transformants retain the plasmid throughout cell divisions. such that it can be recovered and used in other applications. Without functional RecA protein the exogenous plasmid DNA is left unaltered by the bacteria. Purification of this plasmid from bacterial cultures then results in high-fidelity amplification of the original plasmid sequence.

RecA as a Drug Target

Recently, Wigle and Singleton at the University of North Carolina have shown that small molecules interfering with RecA function in the cell may be useful in the creation of new pharmaceuticals. Since many antibiotics lead to DNA damage, and all bacteria rely on RecA to fix this damage, inhibitors of RecA could be used to enhance the toxicity of antibiotics. Additionally the activities of RecA are synonymous with antibiotic resistance development, and inhibitors of RecA may also serve to delay or prevent the appearance of bacterial drug resistance.

References

  • Joo C, McKinney SA, Nakamura M, Rasnik I, Myong S, Ha T (2006). "Real-time observation of RecA filament dynamics with single monomer resolution". Cell 126 (3): 515–27.

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