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RecA is a 38
kilodalton The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at ...
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
essential for the repair and maintenance of DNA. A RecA structural and functional homolog has been found in every species in which one has been seriously sought and serves as an archetype for this class of homologous DNA repair
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s. The homologous protein is called
RAD51 DNA repair protein RAD51 homolog 1 is a protein encoded by the gene ''RAD51''. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to th ...
in
eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
and
RadA The Royal Academy of Dramatic Art (RADA; ) is a drama school in London, England, that provides vocational conservatoire training for theatre, film, television, and radio. It is based in the Bloomsbury area of Central London, close to the S ...
in
archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...
. RecA has multiple activities, all related to
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
. In the
bacterial Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were amon ...
SOS response The SOS response is a global response to DNA damage in which the cell cycle is arrested and DNA repair and mutagenesis is induced. The system involves the RecA protein (Rad51 in eukaryotes). The RecA protein, stimulated by single-stranded DNA ...
, it has a co-
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
function in the autocatalytic cleavage of the LexA
repressor In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. A DNA-binding repressor blocks the attachment of RNA polymerase to t ...
and the λ repressor. RecA's association with DNA repair is based on its central role in
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
. The RecA protein binds strongly and in long clusters to ssDNA to form a nucleoprotein filament. The protein has more than one DNA binding site, and thus 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 complementary region of single-stranded DNA. The RecA-ssDNA filament searches for sequence similarity along the dsDNA. A disordered DNA loop in RecA, Loop 2, contains the residues responsible for DNA homologous recombination. In some bacteria, RecA posttranslational modification via phosphorylation of a serine residue on Loop 2 can interfere with homologous recombination. The search process induces stretching of the DNA duplex, which enhances sequence complementarity recognition (a mechanism termed
conformational proofreading Conformational proofreading or conformational selection is a general mechanism of molecular recognition systems in which introducing a structural mismatch between a molecular recognizer and its target, or an energetic barrier, enhances the recogn ...
). The reaction initiates the exchange of strands between two recombining DNA double helices. After the synapsis 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. Since it is a DNA-dependent
ATPase ATPases (, Adenosine 5'-TriPhosphatase, adenylpyrophosphatase, ATP monophosphatase, triphosphatase, SV40 T-antigen, ATP hydrolase, complex V (mitochondrial electron transport), (Ca2+ + Mg2+)-ATPase, HCO3−-ATPase, adenosine triphosphatase) are ...
, RecA 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 Adp or ADP may refer to: Aviation * Aéroports de Paris, airport authority for the Parisian region in France * Aeropuertos del Perú, airport operator for airports in northern Peru * SLAF Anuradhapura, an airport in Sri Lanka * Ampara Airp ...
bound. In ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Esc ...
'',
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
events mediated by RecA can occur during the period after
DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritan ...
when sister loci remain close. RecA can also mediate homology pairing, homologous recombination and DNA break repair between distant sister loci that had segregated to opposite halves of the ''E. coli'' cell. ''E. coli'' strains deficient in RecA are useful for
cloning Cloning is the process of producing individual organisms with identical or virtually identical DNA, either by natural or artificial means. In nature, some organisms produce clones through asexual reproduction. In the field of biotechnology, c ...
procedures in
molecular biology Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including biomolecular synthesis, modification, mechanisms, and interactions. The study of chemical and phys ...
laboratories. ''E. coli'' strains are often genetically modified to contain a mutant ''recA''
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
and thereby ensure the stability of extrachromosomal segments of DNA, known as
plasmids A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...
. In a process called transformation, 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 can then allow high-fidelity PCR amplification of the original plasmid sequence.


Potential as a drug target

Wigle and Singleton at the
University of North Carolina The University of North Carolina is the multi-campus public university system for the state of North Carolina. Overseeing the state's 16 public universities and the NC School of Science and Mathematics, it is commonly referred to as the UNC S ...
have shown that small molecules interfering with RecA function in the cell may be useful in the creation of new
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
drugs. 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.


Role of RecA in natural transformation

Based on analysis of the molecular properties of the RecA system, Cox concluded that the data "provide compelling evidence that the primary mission of RecA protein is
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
." In a further essay on the function of the RecA protein, Cox summarized data demonstrating that "RecA protein evolved as the central component of a recombinational
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA d ...
system, with the generation of genetic diversity as a sometimes useful byproduct." Natural bacterial transformation involves the transfer of DNA from one bacterium to another (ordinarily of the same
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
) and the integration of the donor DNA into the recipient chromosome by
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
, a process mediated by the RecA protein (see
Transformation (genetics) In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane(s). For transformation to t ...
). Transformation, in which RecA plays a central role, depends on expression of numerous additional gene products (e.g. about 40 gene products in '' Bacillus subtilis'') that specifically interact to carry out this process indicating that it is an evolved
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
for DNA transfer. In ''B. subtilis'' the length of the transferred DNA can be as great as a third and up to the size of the whole
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
. In order for a bacterium to bind, take up and recombine exogenous DNA into its chromosome, it must first enter a special physiological state termed "competence" (see Natural competence). Transformation is common in the prokaryotic world, and thus far 67 species are known to be competent for transformation. One of the most well studied transformation systems is that of ''B. subtilis''. In this bacterium, the RecA protein interacts with the incoming single-stranded DNA (ssDNA) to form striking filamentous structures. These RecA/ssDNA filaments emanate from the cell pole containing the competence machinery and extend into the cytosol. The RecA/ssDNA filamentous threads are considered to be dynamic nucleofilaments that scan the resident chromosome for regions of homology. This process brings the incoming DNA to the corresponding site in the ''B. subtilis'' chromosome where informational exchange occurs. Michod et al. have reviewed evidence that RecA-mediated transformation is an adaptation for homologous recombinational repair of DNA damage in ''B. subtilis'', as well as in several other bacterial species (i.e. ''
Neisseria gonorrhoeae ''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Neisser in 1879. It causes the sexually transmitted genitourinary infection gon ...
, Hemophilus influenzae, Streptococcus pneumoniae,
Streptococcus mutans ''Streptococcus mutans'' is a facultatively anaerobic, gram-positive coccus (round bacterium) commonly found in the human oral cavity and is a significant contributor to tooth decay. It is part of the "streptococci" (plural, non-italic lowerc ...
'' and '' Helicobacter pylori''). In the case of the pathogenic species that infect humans, it was proposed that RecA-mediated repair of DNA damages may be of substantial benefit when these bacteria are challenged by the oxidative defenses of their host.


References

*{{cite journal , vauthors=Joo C, McKinney SA, Nakamura M, Rasnik I, Myong S, Ha T , title=Real-time observation of RecA filament dynamics with single monomer resolution , journal=Cell , volume=126 , issue=3 , pages=515–27 , date=August 2006 , pmid=16901785 , doi=10.1016/j.cell.2006.06.042 , doi-access=free Bacterial proteins DNA repair