Bacterial conjugation is the transfer of genetic material betweenbacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells.[1] This takes place through apilus.[2][full citation needed] It is aparasexual mode of reproduction in bacteria.
It is a mechanism ofhorizontal gene transfer as aretransformation andtransduction although these two other mechanisms do not involve cell-to-cell contact.[4]
ClassicalE. coli bacterial conjugation is often regarded as the bacterial equivalent ofsexual reproduction ormating, since it involves the exchange of genetic material. However, it is not sexual reproduction, since no exchange of gamete occurs, and indeed nogeneration of a new organism: instead, an existing organism is transformed. During classicalE. coli conjugation, thedonor cell provides a conjugative or mobilizable genetic element that is most often aplasmid ortransposon.[5] Most conjugative plasmids have systems ensuring that therecipient cell does not already contain a similar element.
The genetic information transferred is often beneficial to the recipient. Benefits may includeantibiotic resistance,xenobiotic tolerance or the ability to use newmetabolites.[1] Other elements can be detrimental, and may be viewed as bacterialparasites.
Conjugation inEscherichia coli by spontaneous zygogenesis[6] and inMycobacterium smegmatis by distributive conjugal transfer[7][8] differ from the better studied classicalE. coli conjugation in that these cases involve substantial blending of the parentalgenomes.
The process was discovered byJoshua Lederberg andEdward Tatum[9] in 1946.
Conjugation diagram
TheF-factor is anepisome (a plasmid that can integrate itself into the bacterialchromosome byhomologous recombination) with a length of about 100kb. It carries its ownorigin of replication, theoriV, and an origin of transfer, ororiT.[5] 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 calledF-positive orF-plus (denoted F+). Cells that lack F plasmids are calledF-negative orF-minus (F−) and as such can function as recipient cells.[citation needed]
Among other genetic information, the F-plasmid carries atra andtrblocus, which together are about 33 kb long and consist of about 40genes. Thetra locus includes thepilin gene and regulatory genes, which together formpili on the cell surface. The locus also includes the genes for theproteins 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 the structures through which DNA exchange occurs. The F-pili are extremely resistant to mechanical and thermochemical stress, which guarantees successful conjugation in a variety of environments.[10] Several proteins coded for in thetra ortrb 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.[citation needed]
When conjugation is initiated by a signal, therelaxaseenzyme creates anick in one of the strands of the conjugative plasmid at theoriT. Relaxase may work alone, or in a complex of over a dozen proteins known collectively as arelaxosome. 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, orT-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 theoriV) or in concert with conjugation (conjugative replication similar to therolling circle replication oflambda phage). 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.[11]
If the F-plasmid that is transferred has previously been integrated into the donor's genome (producing an Hfr strain ["High Frequency of Recombination"]) some of the donor's chromosomal DNA may also be transferred with the plasmid DNA.[4] The amount of chromosomal DNA that is transferred depends on how long the two conjugating bacteria remain in contact. In common laboratory strains ofE. coli the transfer of the entire bacterial chromosome takes about 100 minutes. The transferred DNA can then be integrated into the recipient genome viahomologous 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 calledHfr (highfrequency ofrecombination). TheE. coligenome 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.[citation needed]
Since integration of the F-plasmid into theE. coli chromosome is a rare spontaneous occurrence, and since the numerous genes promoting DNA transfer are in the plasmid genome rather than in the bacterial genome, it has been argued that conjugative bacterial gene transfer, as it occurs in theE. coli Hfr system, is not an evolutionary adaptation of the bacterial host, nor is it likely ancestral to eukaryotic sex.[13]
Spontaneous zygogenesis inE. coli
In addition to classical bacterial conjugation described above forE. coli, a form of conjugation referred to as spontaneous zygogenesis (Z-mating for short) is observed in certain strains ofE. coli.[6] In Z-mating there is complete genetic mixing, and unstablediploids are formed that throw off phenotypically haploid cells, of which some show a parentalphenotype and some are truerecombinants.[citation needed]
Conjugation inMycobacteria smegmatis, like conjugation inE. coli, requires stable and extended contact between a donor and a recipient strain, is DNase resistant, and the transferred DNA is incorporated into the recipient chromosome by homologous recombination. However, unlikeE. coli Hfr conjugation, mycobacterial conjugation is chromosome rather than plasmid based.[7][8] Furthermore, in contrast toE. coli Hfr conjugation, inM. smegmatis all regions of the chromosome are transferred with comparable efficiencies. The lengths of the donor segments vary widely, but have an average length of 44.2kb. Since a mean of 13 tracts are transferred, the average total of transferred DNA per genome is 575kb.[8] This process is referred to as "Distributive conjugal transfer."[7][8] Gray et al.[7] found substantial blending of the parental genomes as a result of conjugation and regarded this blending as reminiscent of that seen in the meiotic products of sexual reproduction.
Hyperthermophilicarchaea encode pili structurally similar to the bacterial conjugative pili.[14] However, unlike in bacteria, where conjugation apparatus typically mediates the transfer of mobile genetic elements, such as plasmids or transposons, the conjugative machinery of hyperthermophilic archaea, called Ced (Crenarchaeal system for exchange of DNA)[15] and Ted (Thermoproteales system for exchange of DNA),[14] appears to be responsible for the transfer of cellular DNA between members of the same species. It has been suggested that in these archaea the conjugation machinery has been fully domesticated for promoting DNA repair through homologous recombination rather than spread of mobile genetic elements.[14] In addition to the VirB2-like conjugative pilus, the Ced and Ted systems include components for the VirB6-like transmembrane mating pore and the VirB4-like ATPase.[14]
Bacteria related to thenitrogen fixingRhizobia are an interesting case of inter-kingdom conjugation.[16] For example, the tumor-inducing (Ti) plasmid ofAgrobacterium and the root-tumor inducing (Ri) plasmid ofA. rhizogenes contain genes that are capable of transferring to plant cells. The expression of these genes effectively transforms the plant cells intoopine-producing factories. Opines are used by the bacteria as sources of nitrogen and energy. Infected cells formcrown gall orroot tumors. The Ti and Ri plasmids are thusendosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.[citation needed]
The Ti and Ri plasmids can also be transferred between bacteria using a system (thetra, ortransfer, operon) that is different and independent of the system used for inter-kingdom transfer (thevir, orvirulence, operon). Such transfers create virulent strains from previously avirulent strains.[citation needed]
Conjugation is a convenient means fortransferring genetic material to a variety of targets. In laboratories, successful transfers have been reported from bacteria to yeast,[17] plants, mammalian cells,[18][19]diatoms[20] and isolated mammalianmitochondria.[21] Conjugation has advantages over other forms of genetic transfer including minimal disruption of the target'scellular envelope and the ability to transfer relatively large amounts of genetic material (see the above discussion ofE. coli chromosome transfer). In plant engineering,Agrobacterium-like conjugation complements other standard vehicles such astobacco mosaic virus (TMV). While TMV is capable of infecting many plant families these are primarilyherbaceousdicots.Agrobacterium-like conjugation is also primarily used for dicots, butmonocot recipients are not uncommon.[citation needed]