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Bacterial conjugation

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	Bacterial conjugation
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	Topics Bacterial conjugation Topic Home Bacterial conjugation is the transfer of genetic material between bacteria through direct cell-to-cell contact. Discovered in 1946 by Joshua LederbergJoshua Lederberg Dr. Joshua Lederberg is an American molecular biologist who is known for his work in genetics, artificial intelligence, and ... and Edward Tatum, conjugation is a mechanism of horizontal gene transferHorizontal gene transfer Horizontal gene transfer, also Lateral gene transfer, is any process in which an organism transfers genetic material t... —as are transformationTransformation (genetics) Transformation is the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign genetic... and transductionTransduction (genetics) Transduction is the process by which bacterial DNA is moved from one bacterium to another by a virus.... —although these mechanisms do not involve cell-to-cell contact. Bacterial conjugation is often incorrectly regarded as the bacterial equivalent of sexual reproductionSexual reproduction Sexual reproduction is a type of reproduction that results in increasing genetic diversity of the offspring.... or matingMating Summary In biology, mating is the pairing of opposite-sex or hermaphroditic internal fertilization animals for copulation and insemi... . It is not actually sexual, as it does not involve the fusing of gameteGamete Gametes, from the ancient Greek ?aet?? , are the specialized germ cells that come together during fertilization in orga... s and the creation of a zygoteZygote A zygote is a cell that is the result of fertilization.... , nor is there equal exchange of genetic material. It is merely the transfer of genetic information from a donor cell to a recipient. In order to perform conjugation, one of the bacteria, the donor, must play host to a conjugative or mobilizable genetic element, most often a conjugative or mobilizable plasmidPlasmid A plasmid is a DNA molecule separate from the chromosomal DNA and capable of autonomous replication.... or transposonTransposon Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process c... . Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element. The genetic information transferred is often beneficial to the recipient cell. Benefits may include antibiotic resistanceAntibiotic resistance Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic.... , other xenobioticXenobiotic A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it.... tolerance, or the ability to utilize a new metaboliteMetabolite Metabolites are the intermediates and products of metabolism.... . Such beneficial plasmids may be considered bacterial endosymbionts. Some conjugative elements may also be viewed as genetic parasites on the bacterium, and conjugation as a mechanism was evolved by the mobile element to spread itself into new hosts. Mechanism The prototypePrototype A prototype is an original type, form, or instance of some thing serving as a typical example, basis, epitome, or standard f... for conjugative plasmids is the F-plasmid, also called the F-factor. The F-plasmid is an episome (a plasmid that can integrate itself into the bacterial chromosomeChromosome A chromosome is a large macromolecule into which DNA is normally packaged in a cell.... by genetic recombinationGenetic recombination Genetic recombination is the transmission-genetic process by which the combinations of alleles observed at different loci in... ) of about 100 kb length. It carries its own origin of replicationOrigin of replication The origin of replication is a unique DNA sequence at which DNA replication is initiated.... , the oriV, as well as an origin of transfer, or oriT. There can only be one copy of the F-plasmid in a given bacterium, either free or integrated (two immediately before cell division). The host bacterium is called F-positive or F-plus (denoted F⁺). Strains that lack F plasmids are called F-negative or F-minus (F^-). Among other genetic information, the F-plasmid carries a tra and a trb locusLocus (genetics) In biology and evolutionary computation, a locus is a fixed position on a chromosome, such as the position of a gene.... , which together are about 33 kb long and consist of about 40 geneGene A gene is the unit of heredity in living organisms.... s. The tra locus includes the pilin gene and regulatory genes, which together form piliPilus A pilus is a hairlike structure on the surface of a cell, especially Gram-negative bacteria, a protein appendage required f... on the cell surface, polymeric proteinProtein Proteins are large organic compounds made of amino acids arranged in a linear chain and joined by peptide bonds.... s that can attach themselves to the surface of F^- bacteria and initiate the conjugation. Though there is some debate on the exact mechanism, the pili themselves do not seem to be the structures through which the actual exchange of DNA takes place; rather, some proteins coded in the tra or trb loci seem to open a channel between the bacteria. When conjugation is initiated, via a mating signal, a relaxase enzyme creates a nickNick (DNA) A nick is a point in a double stranded DNA molecule where there is no phosphodiester bond between adjacent nucleotides of on... in one plasmid DNA strand at the origin of transfer, or oriT. The relaxase may work alone or in a complex of over a dozen proteins, known collectively as a relaxosome. In the F-plasmid system, the relaxase enzyme is called TraI and the relaxosome consists of TraI, TraY, TraM, and the integrated host factor, IHF. The transferred, or T-strand, is unwound from the duplex plasmid and transferred into the recipient bacterium in a 5'-terminus to 3'-terminus direction. The remaining strand is replicated, either independent of conjugative action (vegetative replication, beginning at the oriV) or in concert with conjugation (conjugative replication similar to the rolling circle replication of lambda phageLambda phage *Enterobacteria phage ?* is a temperate bacteriophage that infects Escherichia coli.... ). Conjugative replication may necessitate a second nick before successful transfer can occur. A recent report claims to have inhibited conjugation with chemicals that mimic an intermediate step of this second nicking event. If the F-plasmid becomes integrated into the host genome, donor chromosomal DNA may be transferred along with plasmid DNA. The certain amount of chromosomal DNA that is transferred depends on how long the bacteria remain in contact; for common laboratory strains of E. coliEscherichia coli *Escherichia coli* , usually abbreviated to E.... the transfer of the entire bacterial chromosome takes about 100 minutes. The transferred DNA can be integrated into the recipient genome via recombination. A culture of cells containing non-integrated F plasmids usually contains a few that have accidentally become integrated, and these are responsible for those low-frequency chromosomal gene transfers which do occur in such cultures. Some strains of bacteria with an integrated F-plasmid can be isolated and grown in pure culture. Because such strains transfer chromosomal genes very efficiently, they are called HfrHfr cell A hfr cell is a bacterium with a conjugative plasmid integrated into its genomic DNA.... (high frequency of recombination). The E. coli genomeGenome In biology the genome of an organism is its whole hereditary information and is encoded in the DNA .... was originally mapped by interrupted mating experiments, in which various Hfr cells in the process of conjugation were sheared from recipients after less than 100 minutes (initially using a Waring blender) and investigating which genes were transferred. Inter-kingdom transfer The nitrogen fixingDiazotroph Diazotrophs are bacteria that fix atmospheric nitrogen gas into a more usable form such as ammonia.... RhizobiaRhizobia Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes.... are an interesting case, wherein conjugative elements naturally engage in inter-kingdomKingdom (biology) In biology, a kingdom or regnum is the top-level, or nearly the top-level, taxon of organisms in scientific classifica... conjugation. Such elements as the AgrobacteriumAgrobacterium Agrobacterium is a genus of bacteria that causes tumors in plants.... Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of opinesOpines Opines are low molecular weight compounds found in plant crown gall tumors produced by the parasitic bacterium Agrobacteri... , which the bacteria use as carbon and energy sources. Infected plant cells form crown gallAgrobacterium tumefaciens *Agrobacterium tumefaciens* is a species of bacteria that causes tumors in dicots.... or root tumorsAgrobacterium rhizogenes Agrobacterium rhizogenes is a Gram negative soil bacterium.... . The Ti and Ri plasmids are thus endosymbiontEndosymbiont An endosymbiont is any organism that lives within the body or cells of another organism, i.e.... s of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant. The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (the tra, or transfer, operon) from that for inter-kingdom transfer (the vir, or virulenceVirulence Virulence is either the relative pathogenicity or the relative ability to do damage to the host of an infectious agent.... , operon). Such transfer creates virulent strains from previously avirulent Agrobacteria. See also TransfectionTransfection Transfection is the introduction of DNA into eukaryotic cells, such as animal cells.... Transduction Triparental matingTriparental mating Triparental mating is used for the introduction of plasmids... External links Transgenic poplars: Is horizontal gene transfer from Agrobacteria to endophytic bacteria possible?