Definition of bacterial conjugation
Bacterial conjugation is a way that a bacterial cell transfers genetic material to another bacterial cell. The genetic material that is transferred through bacterial conjugation is a small plasmid, known as an F-plasmid (F for fertility factor), which carries different genetic information than what is already present in the bacterial cell’s chromosomes. In fact, the F plasmid can replicate in the cytoplasm separately from the bacterial chromosome.
A cell that already has a copy of the F plasmid is called an F-positive, F-plus, or F+ cell, and is considered a donor cell, while a cell that does not have a copy of the F plasmid is called an F-negative cell, F-minus or F–, and is considered a receptor cell. The transfer of the F plasmid is carried out through a horizontal connection whereby the donor cell and the recipient cell directly contact each other or form a bridge between them through which the genetic material is transferred. In cases where the F plasmid from a donor cell has integrated into the cell’s genome (i.e., into the chromosome), a portion of the chromosomal DNA may also be transferred to the recipient cell along with the F plasmid.
Bacterial conjugation steps
To transfer the F plasmid, a donor cell and a recipient cell must first make contact. At this point, when the cells make contact, the F plasmid in the donor cell is a double-stranded DNA molecule that forms a circular structure. The following steps allow the transfer of the F plasmid from one bacterial cell to another:
The F+ (donor) cell produces the pilus, which is a structure that projects out of the cell and begins contact with an F– (recipient) cell.
The pilus allows direct contact between donor and recipient cells.
Because the F plasmid consists of a double-stranded DNA molecule that forms a circular structure, that is, it is joined at both ends, an enzyme (relaxase or relaxosome when it forms a complex with other proteins) cuts one of the two chains. of DNA. from the F plasmid and this chain (also called the T chain) is transferred to the recipient cell.
In the last step, the donor cell and the recipient cell, both with single-stranded DNA, replicate this DNA and thus end up forming a double-stranded F-plasmid identical to the original F-plasmid. Since the F plasmid contains information to synthesize pili and other proteins, the former recipient cell is now a donor cell with the F plasmid and the ability to form pili, just as the original donor cell was. Now both cells are donors or F+.
To prevent the transfer of the F plasmid to an F+ cell, the F plasmid often contains information that allows the donor cell to detect (and avoid) cells that already have one. Furthermore, plasmid F contains two major loci (tra and trb), an origin of replication (Orig), and an origin of transfer (Orig). The tra locus contains the genetic information to allow the donor cell to attach to a recipient cell: genes at the tra locus encode proteins to form pili (pilin gene) to initiate cell-cell contact and other proteins to attach to the F– cell and begin the transfer of the F plasmid. The trb locus contains DNA that encodes other proteins, including some that are involved in creating a channel through which DNA is transferred from the F+ cell to the F–. The Orig is the site where DNA replication occurs and the OriT is the site where the relaxase enzyme (or relaxosome protein complex) cuts the DNA strand of the F plasmid (see step 3 above).
Although the DNA that is transferred in bacterial conjugation is that present on the F plasmid when the donor cell has integrated the F plasmid into its own chromosomal DNA, bacterial conjugation can result in the transfer of the F plasmid and chromosomal DNA. . When this is the case, longer contact between donor and recipient cells results in a greater amount of chromosomal DNA being transferred.
The advantages of bacterial conjugation make this method of gene transfer a widely used technique in bioengineering. Some of the advantages include the ability to transfer relatively large DNA sequences and not damage the host cell envelope. Furthermore, conjugation has been achieved in laboratories not only between bacteria, but also between bacteria and cell types such as plant cells, mammalian cells, and yeast.