Conjugation is the transfer of a plasmid, other self-transmissible DNA element, and in some cases chromosomal DNA, from a donor cell to a recipient cell through direct contact, usually mediated by a conjugation pilus or sex pilus.
The recipient cells that acquire DNA through conjugation are referred to as transconjugants.
This process is capable of transferring DNA regions ranging from hundreds to thousands of kilobases and represents the method of bacterial exchange with the broadest host range for DNA transfer.
Conjugation occurs both within species and between different species of bacteria, including Gram-negative and Gram-positive bacteria, and it can even take place between bacteria and plants.
Broad-host-range conjugative plasmids have been utilized in molecular biology as tools to introduce recombinant genes into bacterial species that are resistant or difficult to manipulate using standard transformation or transduction methods.
While there are numerous examples of conjugative plasmids, the most commonly studied and described form of conjugation involves the F plasmid.
Principle of Bacterial Conjugation
The principle of bacterial conjugation is that a plasmid or other genetic material is transferred from a donor cell to a recipient cell through close physical contact.
Among all conjugative plasmids, the F (fertility) plasmid of E. coli was the first to be discovered and remains one of the best-studied examples.
The F plasmid is typically present in one or two copies per cell and is relatively large, about 100 kilobases in size.
E. coli cells carrying the F plasmid are called donor cells (F⁺ or male), while E. coli cells lacking the F plasmid are referred to as recipient cells (F⁻ or female).
Only donor (F⁺) cells have the ability to transfer the F plasmid to recipient (F⁻) cells.
The transfer of the F plasmid requires intimate contact between the donor and recipient, leading to the formation of a mating pair.
The genetic material is then transferred through membrane fusion between the two cells, facilitated by the action of specific enzymes.
After membrane fusion, replication of the donor DNA takes place, and the newly synthesized DNA is transferred into the recipient cell.
Steps/Process of Bacterial Conjugation
During conjugation in E. coli, the F plasmid plays a central role in the transfer process.
The F plasmid carries the tra locus, which includes the pilin gene along with regulatory proteins responsible for producing pili on the surface of the F⁺ (donor) cell.
The proteins of the pili attach to the surface of the F⁻ (recipient) cell. The pili function to establish contact between the two cells; however, the actual plasmid transfer does not occur through the pili.
At the base of the pilus, the enzyme TraD initiates membrane fusion between the donor and recipient cells.
Once conjugation begins, the enzyme relaxase introduces a nick at the origin of transfer (oriT) in the conjugative plasmid.
The nicked single strand, known as the T strand, unwinds and is transferred into the recipient cell in the 5′ → 3′ direction.
Both donor and recipient cells then synthesize complementary strands, resulting in each cell possessing a complete F plasmid; thus, both cells become F⁺.
Occasionally, in about 1 out of every 10,000 F⁺ cells, the F element integrates into the bacterial chromosome. In such cases, a portion of the bacterial chromosome is transferred instead of just the plasmid.
During this integration process, the circular F element breaks at a specific site and inserts as a linear segment into the bacterial chromosome.
A cell containing such an integrated F element is termed an Hfr cell (High Frequency of Recombination).
In Hfr cells, the integrated F element is usually replicated passively along with the bacterial chromosome, ensuring its stable inheritance and transmission from one Hfr generation to the next.
Other conjugative elements
Broad-host-range conjugative plasmids such as RK2 are capable of transferring across a wide variety of bacterial genera and, in some cases, even from bacteria to yeast.
Some plasmids contain an origin of transfer (oriT) but are not self-transmissible, as they lack some or all of the essential tra genes required for transfer.
When the tra genes are supplied by another replicon, these otherwise non-transmissible plasmids can be mobilized and transferred. Such plasmids are termed mobilizable plasmids.
Examples of bacterial conjugation
An example of bacterial conjugation is seen in Agrobacterium tumefaciens, which causes crown gall tumors in plants. This occurs when the bacterium transfers the T-DNA element, a segment of its Ti (tumor-inducing) plasmid, into plant cells, where the T-DNA becomes integrated into the plant genome.
Conjugative plasmids encoding antimicrobial resistance genes are known as R plasmids.
These R plasmids can be transferred between pathogenic bacteria, such as Shigella spp., leading to the emergence and spread of antibiotic-resistant strains and potentially causing widespread outbreaks of Shigella-mediated dysentery.
References
Verma, P. S., & Agarwal, V. K. (2005). Cell Biology, Genetics, Molecular Biology, Evolution and Ecology (Multicolored Edition).
McGee, D., Coker, C., Harro, J., & Mobley, H. (2001). Bacterial genetic exchange. Nature Encyclopedia of Life Sciences. https://doi.org/10.1038/npg.els.0001416
Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., et al. (2000). An Introduction to Genetic Analysis (7th ed.). New York: W. H. Freeman. Available at: https://www.ncbi.nlm.nih.gov/books/NBK21942/
Adelberg, E. A., & Pittard, J. (1965). Chromosome transfer in bacterial conjugation. Bacteriological Reviews.
Deng, Y., Zhang, X., & Zhang, X. (2017). Recent advances in genetic modification systems for Actinobacteria. Applied Microbiology and Biotechnology, 101(6), 2217–2226. https://doi.org/10.1007/s00253-017-8156-1
