Bacterial Conjugation- Definition, Principle, Process, Examples
Bacteria are microscopic organisms that can reproduce by binary fission, a process that produces genetically identical copies of the parent cell. However, bacteria can also exchange genetic material with other bacteria by different mechanisms, such as transformation, transduction, and conjugation. These mechanisms increase the genetic diversity and adaptability of bacterial populations, and can also transfer important traits such as antibiotic resistance or virulence.
Among these mechanisms, bacterial conjugation is the only one that requires direct contact between two bacterial cells. Conjugation involves the transfer of a plasmid or other self-transmissible DNA element, and sometimes chromosomal DNA, from a donor cell to a recipient cell via a specialized structure called a pilus or sex pilus. The recipient cell that receives the DNA by conjugation is called a transconjugant.
Conjugation is a parasexual mode of reproduction in bacteria, meaning that it does not involve meiosis or gamete formation. Conjugation is also universally conserved among bacteria and occurs in a wide range of environments. It can even occur between bacteria and plants, such as in the case of Agrobacterium tumefaciens, which transfers a part of its plasmid to plant cells and causes crown gall tumors.
The most common and well-studied example of bacterial conjugation is the transfer of the F (fertility) plasmid in Escherichia coli. The F plasmid contains genes that encode for the pilus formation and DNA transfer, as well as other functions. Bacteria that carry the F plasmid are called donor (F+) cells, and bacteria that lack the F plasmid are called recipient (F-) cells. Only F+ cells can initiate conjugation with F- cells and transfer the F plasmid to them.
In some cases, the F plasmid can integrate into the bacterial chromosome and form an Hfr (high frequency of recombination) cell. An Hfr cell can transfer a part of its chromosome along with the F plasmid to an F- cell by conjugation. This results in genetic recombination between the donor and recipient chromosomes.
Besides the F plasmid, there are other types of conjugative elements that can mediate gene transfer between bacteria. For instance, R (resistance) plasmids can confer resistance to antibiotics or other agents by conjugation. Some plasmids are not self-transmissible but can be mobilized by other conjugative elements that provide the necessary genes for pilus formation and DNA transfer.
Bacterial conjugation is a fascinating phenomenon that has many implications for bacterial evolution, ecology, and biotechnology. In this article, we will explore the definition, principle, process, and examples of bacterial conjugation in more detail.
Conjugation is a type of horizontal gene transfer that involves the direct transfer of DNA from one bacterial cell to another through a physical connection called a conjugation pilus or sex pilus. The DNA that is transferred can be a plasmid, a circular piece of DNA that can replicate independently of the chromosome, or a part of the chromosome itself. The donor cell usually contains a special type of plasmid called a conjugative plasmid that encodes the genes for pilus formation and DNA transfer. The recipient cell does not have the conjugative plasmid and is usually of the same or closely related species as the donor. The recipient cell that receives the DNA from the donor is called a transconjugant.
Conjugation is one of the main mechanisms by which bacteria exchange genetic information and acquire new traits such as antibiotic resistance, virulence factors, metabolic pathways, and biodegradation abilities. Conjugation can occur in both Gram-positive and Gram-negative bacteria, as well as between bacteria and plants. Conjugation can also be used as a tool for genetic engineering and molecular biology to introduce recombinant DNA into bacterial cells that are otherwise difficult to transform or transduce.
The process of conjugation varies depending on the type of plasmid and the bacterial species involved, but it generally involves the following steps:
- The donor cell initiates contact with the recipient cell by extending a conjugation pilus that binds to a specific receptor on the recipient cell surface.
- The pilus retracts and brings the two cells into close contact, forming a mating pair.
- The donor cell transfers a single-stranded copy of the plasmid or chromosomal DNA to the recipient cell through a pore or channel that connects the cytoplasms of both cells.
- The transferred DNA strand is replicated in both cells by using the complementary strand as a template, resulting in two double-stranded circular molecules of DNA in each cell.
- The mating pair separates and both cells resume normal growth and division.
The outcome of conjugation depends on the type and size of the DNA that is transferred. If the transferred DNA is a plasmid, both cells will become plasmid-bearing and will have the same traits encoded by the plasmid. If the transferred DNA is a part of the chromosome, only the recipient cell will acquire new traits from the donor, while the donor cell will lose some of its original traits. The transferred chromosomal DNA may integrate into the recipient`s chromosome by homologous recombination or may remain as an extrachromosomal element.
Bacterial conjugation is a process of genetic exchange between bacteria that involves direct cell-to-cell contact mediated by a specialized structure called a pilus. The pilus is encoded by a plasmid or other self-transmissible DNA element that can be transferred from a donor cell to a recipient cell. The recipient cell that receives the DNA by conjugation is called a transconjugant.
Bacterial conjugation can transfer large regions of DNA, ranging from hundreds to thousands of kilobases, and has a broad host range that includes Gram-negative and Gram-positive bacteria, and even plants. Bacterial conjugation plays an important role in the evolution and adaptation of bacterial populations by spreading various traits, such as antibiotic resistance, virulence factors, metabolic capabilities, and genetic diversity.
The most common and well-studied example of bacterial conjugation is the transfer of the F (fertility) plasmid in E. coli. The F plasmid is a large (about 100 kb) circular DNA molecule that confers the ability to form a pilus and initiate conjugation. E. coli cells that carry the F plasmid are called donor (F+ or male) cells, and those that lack the F plasmid are called recipient (F- or female) cells. Only F+ cells can transfer the F plasmid to F- cells, resulting in two F+ cells after conjugation.
The principle underlying bacterial conjugation is that the plasmid or other genetic material is transferred from the donor cell to the recipient cell via intimate physical contact. The F (fertility) plasmid of E. coli was the first conjugative plasmid discovered and is one of the most thoroughly studied.
The process of bacterial conjugation involves the following steps:
- Pilus formation: The donor cell produces a thin, tubelike structure called a pilus (or sex pilus) that extends from its surface and attaches to the recipient cell. The pilus is encoded by the tra genes on the conjugative plasmid (such as the F plasmid) present in the donor cell.
- Cell contact: The pilus contracts and draws the two cells together, forming a stable mating pair. A pore or channel forms at the site of contact, allowing the transfer of DNA between the cells.
- DNA transfer: The conjugative plasmid is nicked at a specific site (called oriT) by an enzyme called relaxase. The relaxase remains bound to the 5` end of the nicked strand (called the T strand) and guides it into the recipient cell through the pore. The remaining strand (called the R strand) in the donor cell is replicated by a rolling circle mechanism. In the recipient cell, the T strand is also replicated to form a circular plasmid identical to the original one in the donor cell. As a result, both cells become F+ and can act as donors in future conjugation events.
- Cell separation: After the completion of DNA transfer and replication, the pore closes and the cells separate. The transferred DNA may recombine with the recipient`s chromosome or remain as an independent plasmid.
The F plasmid, also known as the fertility factor, is a circular DNA molecule that confers the ability to undergo conjugation to E. coli cells that carry it. These cells are called F+ or donor cells, and they can transfer a copy of the F plasmid to F- or recipient cells that lack it. The transfer of the F plasmid involves the following steps:
- The F plasmid contains a set of genes called tra (transfer) that encode the proteins required for conjugation, such as the sex pilus, the relaxase enzyme, and the coupling protein.
- The sex pilus is a thin, hair-like appendage that extends from the surface of the donor cell and attaches to the recipient cell. The pilus then retracts, bringing the two cells into close contact.
- The relaxase enzyme creates a nick at a specific site on the F plasmid called oriT (origin of transfer), and initiates the unwinding and transfer of one strand of the plasmid DNA to the recipient cell through a channel formed by the coupling protein.
- The transferred strand is called the T (transfer) strand, and it moves in the 5`-3` direction. As it enters the recipient cell, it is coated with single-stranded DNA binding proteins to prevent degradation.
- The complementary strand is synthesized in both cells by DNA polymerase, using the remaining strand as a template. Thus, both cells end up with a complete copy of the F plasmid and become F+.
The transfer of the F plasmid is an example of unidirectional gene transfer, as only the donor cell can initiate conjugation and only the plasmid DNA is transferred. However, in some cases, the F plasmid can integrate into the bacterial chromosome by homologous recombination, forming an Hfr (high frequency of recombination) cell. An Hfr cell can transfer both plasmid and chromosomal DNA to an F- cell during conjugation, resulting in genetic recombination.
The transfer of the F plasmid by conjugation has important implications for bacterial evolution and adaptation, as it can spread genes that confer beneficial traits such as antibiotic resistance, virulence factors, or metabolic capabilities among different strains or species of bacteria.
An Hfr cell (high-frequency recombination cell) is a bacterial cell that contains an F plasmid integrated into its chromosome. The F plasmid is a conjugative plasmid that can transfer a copy of itself to another cell through a sex pilus. However, when the F plasmid is integrated into the chromosome, it can also transfer a part of the chromosomal DNA to the recipient cell.
The role of Hfr cells in conjugation is to increase the genetic diversity and recombination of bacterial populations by transferring chromosomal genes along with the F plasmid. The process of Hfr conjugation is as follows:
- The Hfr cell forms a sex pilus with an F- cell (a cell that lacks the F plasmid) and initiates the transfer of DNA.
- A nick is made at a specific site on the integrated F plasmid, called the origin of transfer (oriT), and a single strand of DNA begins to unwind and move into the recipient cell.
- The DNA strand that is transferred includes a part of the F plasmid and a part of the chromosome, depending on how long the conjugation lasts. The transfer of the entire chromosome takes about 100 minutes.
- The recipient cell synthesizes a complementary strand for the transferred DNA, while the donor cell repairs the nicked strand.
- The transferred DNA may recombine with the recipient`s chromosome by homologous recombination, replacing some of the recipient`s genes with those from the donor. This results in new genetic combinations and variations in the bacterial population.
- The recipient cell usually remains an F- cell, unless the entire F plasmid is transferred, which is rare. The donor cell remains an Hfr cell, unless it loses the integrated F plasmid by excision.
Hfr conjugation is different from F+ conjugation, where only the F plasmid is transferred from an F+ cell to an F- cell, without any chromosomal DNA. Hfr conjugation is also different from transduction, where bacterial DNA is transferred by a bacteriophage (a virus that infects bacteria). Hfr conjugation is one of the main mechanisms of horizontal gene transfer in bacteria, which allows them to adapt to changing environments and acquire new traits such as antibiotic resistance.
Besides the F plasmid, there are other types of conjugative elements that can transfer DNA between bacteria. Some of these are:
- Broad-host-range conjugative plasmids: These are plasmids that can be transferred among many bacterial genera and even from bacteria to yeast. For example, the RK2 plasmid can be transferred from E. coli to Pseudomonas, Agrobacterium, Rhizobium, and Saccharomyces.
- Mobilizable plasmids: These are plasmids that have an origin of transfer (oriT) but lack some or all of the genes required for conjugation (tra genes). However, if the tra genes are provided by another plasmid or by the bacterial chromosome, these plasmids can be mobilized for transfer. For example, the ColE1 plasmid can be mobilized by the F plasmid.
- Integrative and conjugative elements (ICEs): These are genetic elements that can integrate into the bacterial chromosome and excise to form a circular molecule that can be transferred by conjugation. ICEs often carry genes for antibiotic resistance, virulence, or metabolism. For example, the SXT element in Vibrio cholerae can confer resistance to sulfamethoxazole, trimethoprim, chloramphenicol, and streptomycin.
Bacterial conjugation can occur in different ways depending on the type of plasmid or genetic element involved and the bacterial species. Here are some examples of bacterial conjugation:
- Agrobacterium tumefaciens causes crown gall tumor in plants by transferring the T DNA element, a part of the Ti (tumor-inducing) plasmid present in this bacterium, into a plant cell where the T element becomes incorporated into the plant cell’s genome. This process allows the bacterium to manipulate the plant metabolism and produce nutrients for itself.
- Conjugative plasmids encoding antimicrobial resistance genes are called R plasmids which are transferred through different bacterial species, such as Shigella spp, Salmonella spp, E. coli, etc. This can result in a widespread outbreak of antibiotic-resistant infections and pose a serious threat to public health.
- Bacteroides fragilis is a Gram-negative anaerobic bacterium that inhabits the human gut and can cause opportunistic infections. This bacterium can transfer its conjugative transposon (CTn), a mobile genetic element that can integrate into the host chromosome, to other bacteria by conjugation. The CTn carries genes for antibiotic resistance, virulence factors, and metabolic enzymes.
- Rhizobium leguminosarum is a Gram-negative bacterium that forms a symbiotic relationship with leguminous plants and fixes atmospheric nitrogen. This bacterium can transfer its symbiotic plasmid (Sym) to other bacteria by conjugation. The Sym plasmid carries genes for nodulation (formation of root nodules) and nitrogen fixation.
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