Bacterial conjugation
Overview
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. Discovered in 1946 by Joshua Lederberg
Joshua Lederberg
Joshua Lederberg ForMemRS was an American molecular biologist known for his work in microbial genetics, artificial intelligence, and the United States space program. He was just 33 years old when he won the 1958 Nobel Prize in Physiology or Medicine for discovering that bacteria can mate and...

 and Edward Tatum, conjugation is a mechanism of horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...

 as are transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

 and transduction
Transduction (genetics)
Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...

 although these two other mechanisms do not involve cell-to-cell contact.

Bacterial conjugation is often incorrectly regarded as the bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l equivalent of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

 or mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

 since it involves the exchange of genetic material.
Encyclopedia
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. Discovered in 1946 by Joshua Lederberg
Joshua Lederberg
Joshua Lederberg ForMemRS was an American molecular biologist known for his work in microbial genetics, artificial intelligence, and the United States space program. He was just 33 years old when he won the 1958 Nobel Prize in Physiology or Medicine for discovering that bacteria can mate and...

 and Edward Tatum, conjugation is a mechanism of horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...

 as are transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

 and transduction
Transduction (genetics)
Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...

 although these two other mechanisms do not involve cell-to-cell contact.

Bacterial conjugation is often incorrectly regarded as the bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l equivalent of sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

 or mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...

 since it involves the exchange of genetic material. During conjugation the donor cell provides a conjugative or mobilizable genetic element that is most often a plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

 or transposon
Transposon
Transposable elements are sequences of DNA that can move or transpose themselves to new positions within the genome of a single cell. The mechanism of transposition can be either "copy and paste" or "cut and paste". Transposition can create phenotypically significant mutations and alter the cell's...

. 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. Benefits may include antibiotic resistance
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...

, xenobiotic
Xenobiotic
A xenobiotic is a chemical which is found in an organism but which is not normally produced or expected to be present in it. It can also cover substances which are present in much higher concentrations than are usual...

 tolerance or the ability to use new metabolites. Such beneficial plasmids may be considered bacterial endosymbionts. Other elements, however, may be viewed as bacterial parasites
Parasitism
Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

 and conjugation as a mechanism evolved by them to allow for their spread.

Mechanism

The prototypical conjugative plasmid is the F-plasmid, or F-factor. The F-plasmid is an episome (a plasmid that can integrate itself into the bacterial chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

 by homologous recombination) with a length of about 100 kb. It carries its own origin of replication
Origin of replication
The origin of replication is a particular sequence in a genome at which replication is initiated. This can either be DNA replication in living organisms such as prokaryotes and eukaryotes, or RNA replication in RNA viruses, such as double-stranded RNA viruses...

, the oriV, and an origin of transfer, or oriT. There can only be one copy of the F-plasmid in a given bacterium, either free or integrated, and bacteria that possess a copy are called F-positive or F-plus (denoted F+). Cells that lack F plasmids are called F-negative or F-minus (F-) and as such can function as recipient cells.

Among other genetic information the F-plasmid carries a tra and trb locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

, which together are about 33 kb long and consist of about 40 gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s. The tra locus includes the pilin gene and regulatory genes, which together form pili
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...

 on the cell surface. The locus also includes the genes for the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s that attach themselves to the surface of F- bacteria and initiate conjugation. Though there is some debate on the exact mechanism of conjugation it seems that the pili are not the structures through which DNA exchange occurs. This has been shown in experiments where the pilus are allowed to make contact, but then are denatured with SDS and yet DNA transformation still proceeds. Several proteins coded for in the tra or trb locus seem to open a channel between the bacteria and it is thought that the traD enzyme, located at the base of the pilus, initiates membrane fusion.

When conjugation is initiated by a signal the relaxase enzyme creates a nick
Nick (DNA)
A nick is a discontinuity in a double stranded DNA molecule where there is no phosphodiester bond between adjacent nucleotides of one strand typically through damage or enzyme action. Nick allows for much needed release of torsion in the strand....

 in one of the strands of the conjugative plasmid at the oriT. 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 nicked strand, or T-strand, is then unwound from the unbroken strand and transferred to the recipient cell 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 phage
Lambda phage
Enterobacteria phage λ is a temperate bacteriophage that infects Escherichia coli.Lambda phage is a virus particle consisting of a head, containing double-stranded linear DNA as its genetic material, and a tail that can have tail fibers. The phage particle recognizes and binds to its host, E...

). Conjugative replication may require 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 that is transferred has previously been integrated into the donor’s genome some of the donor’s chromosomal DNA may also be transferred with the plasmid DNA. The amount of chromosomal DNA that is transferred depends on how long the two conjugating bacteria remain in contact. In common laboratory strains of E. coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

the transfer of the entire bacterial chromosome takes about 100 minutes. The transferred DNA can then be integrated into the recipient genome via homologous recombination.

A cell culture that contains in its population cells with non-integrated F-plasmids usually also contains a few cells that have accidentally integrated their plasmids. It is these cells that are responsible for the low-frequency chromosomal gene transfers that 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 Hfr
Hfr cell
An Hfr cell is a bacterium with a conjugative plasmid integrated into its genomic DNA. Hfr is the abbreviation for high frequency recombination, which was first characterized by Luca Cavalli-Sforza...

 (high frequency of recombination). The E. coli genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

 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). The genes that were transferred were then investigated.

Inter-kingdom transfer

The nitrogen fixing
Diazotroph
Diazotrophs are bacteria and archaea that fix atmospheric nitrogen gas into a more usable form such as ammonia.A diazotroph is an organism that is able to grow without external sources of fixed nitrogen. Examples of organisms that do this are rhizobia and Frankia and Azospirillum. All diazotrophs...

 Rhizobia
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...

are an interesting case of inter-kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...

 conjugation. For example, the tumor-inducing (Ti) plasmid of Agrobacterium
Agrobacterium
Agrobacterium is a genus of Gram-negative bacteria established by H. J. Conn that uses horizontal gene transfer to cause tumors in plants. Agrobacterium tumefaciens is the most commonly studied species in this genus...

and the root-tumor inducing (Ri) plasmid of A. rhizogenes contain genes that are capable of transferring to plant cells. The expression of these genes effectively transforms the plant cells into opine
Opines
Opines are low molecular weight compounds found in plant crown gall tumors or hairy root tumors produced by parasitic bacteria of the genus Agrobacterium. Opine biosynthesis is catalyzed by specific enzymes encoded by genes contained in a small segment of DNA , which is part of the Ti plasmid,...

-producing factories. Opines are used by the bacteria as sources of nitrogen and energy. Infected cells form crown gall
Agrobacterium tumefaciens
Agrobacterium tumefaciens is the causal agent of crown gall disease in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium...

 or root tumors
Agrobacterium rhizogenes
Agrobacterium rhizogenes is a Gram negative soil bacterium that produces hairy root disease in dicotyledonous plants.A...

, respectively. The Ti and Ri plasmids are thus endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...

s of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.

The Ti and Ri plasmids can also be transferred between bacteria using a system (the tra, or transfer, operon) that is different and independent of the system used for inter-kingdom transfer (the vir, or virulence
Virulence
Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

, operon). Such transfers create virulent strains from previously avirulent strains.

Genetic Engineering Applications

Conjugation is a convenient means for transferring genetic material
Genetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...

 to a variety of targets. In laboratories successful transfers have been reported from bacteria to yeast, plants, mammalian cells and isolated mammalian mitochondria. Conjugation has advantages over other forms of genetic transfer including minimal disruption of the target's cellular envelope and the ability to transfer relatively large amounts of genetic material (see the above discussion of E. coli chromosome transfer). In plant engineering, Agrobacterium-like conjugation complements other standard vehicles such as tobacco mosaic virus
Tobacco mosaic virus
Tobacco mosaic virus is a positive-sense single stranded RNA virus that infects plants, especially tobacco and other members of the family Solanaceae. The infection causes characteristic patterns on the leaves . TMV was the first virus to be discovered...

 (TMV). While TMV is capable of infecting many plant families these are primarily herbaceous
Herbaceous plant
A herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...

 dicots. Agrobacterium-like conjugation is also primarily used for dicots, but monocot recipients are not uncommon.

See also

  • Transduction
    Transduction (genetics)
    Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...

  • Transfection
    Transfection
    Transfection is the process of deliberately introducing nucleic acids into cells. The term is used notably for non-viral methods in eukaryotic cells...

  • Transformation
    Transformation (genetics)
    In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

  • Triparental mating
    Triparental mating
    Triparental mating is a form of Bacterial conjugation where a conjugative plasmid present in one bacterial strain assists the transfer of a mobilizable plasmid present in a second bacterial strain into a third bacterial strain. Plasmids are introduced into bacteria for such purposes as...


External links

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