Bacterial conjugation is a gene transfer mechanism, first introduced by the scientists named Lederberg and Tatum in the year 1946. The conjugation method was first studied in Escherichia coli. The formation of a conjugation tube is the characteristic feature in a conjugation mechanism. Instead of transduction and transformation, some bacteria passes the genetic information from a donor to a recipient cell via the cytoplasmic bridge, what we call as “Conjugation tube”.
The bacterial cells (donor and recipient) come closer to each other and eventually form a bridge refers as pilus or conjugation tube that provides a passage for the transfer of genetic material.
Content: Bacterial Conjugation
Meaning of Bacterial Conjugation
Bacterial conjugation is a process, where a donor cell (having fertility factor) comes in close contact with the recipient cell by forming a protuberance called conjugation tube that passes on the genetic material from one cell to other. In a conjugation process, a donor or F+ cell tends to form a conjugation tube, by the presence of self-transmissible F-plasmid. The F-plasmid stands for fertility plasmid that exists in gram-positive and negative bacterial cells.
A recipient or F– cell possessing one strand of the F-plasmid from the donor cell refers as “Transconjugant”. The transfer mechanism of conjugation occurs between the known species or intraspecific. Some bacteria possess promiscuous plasmid that allows genetic exchange within the unrelated species.
It also refers as Fertility factor, which is a combination of three elements like:
Loci gene: A fertility plasmid comprises of mainly two loci, namely tra and trb gene. A transacting or tra gene contains the genome DNA that encodes for the proteins that form a channel for the DNA transfer. Trb gene encodes the proteins that are involved in processes like:
- Cell to cell contact: Encodes protein that helps in the formation of pili around the cell.
- Cell attachment: Encodes protein that helps in the attachment of donor bacterium to the recipient bacterium.
- Transfer: Encodes protein that initiates the exchange of chromosomal DNA.
OriT: It is an acronym of a term named as Origin of transfer. OriT functions as an active site for the migration of chromosomal DNA from a donor bacterium to a recipient bacterium.
Mechanism of Bacterial Conjugation
The mechanism of conjugation includes the following steps:
- Mating pair formation: A donor cell which having self-transmissible plasmid will form a pilus that helps in attachment with the recipient bacterial cell.
- Signalling by coupling protein: Coupling protein complex is a component of Mpf or mating pair formation complex. It provides a signal to create a cut in the chromosomal DNA by activating relaxase enzyme.
- Activation of relaxase: Relaxase activates and binds to the OriT site of the bacterial chromosomal DNA and helps in the strand displacement.
- Chromosomal DNA strand separation: An enzyme helicase helps in the detachment of the chromosomal DNA fragment.
- Plasmid DNA transfer recyclization and replication: The plasmid DNA moves to the recipient cell along with primase enzyme that attaches itself to the free hydroxyl 3’ prime. It has no functional role but completes the replication of the plasmid DNA in a recipient cell.
- Mating pair separation: Finally, the donor and recipient cell detach from one another.
Methods of Conjugation
Three ways can accomplish the transfer of genetic material:
- F+-F– Conjugation
- Hfr-F– Conjugation
- F’-F– Conjugation
This kind of conjugation occurs between the donor cell having Fertility factor (denoted as F+) and the recipient cell that lacks such factor (indicated as F–). F-plasmid can define as the fertility factor that functions in the expression of pilus, synthesis and exchange of plasmid DNA during mating. The role of fertility factor is controlled by the cluster of 25 tra genes, which can be of two types:
Mpf: It is an acronym of the term Mating pair formation. Mpf genes hold the mating cells together and provide a passage for the DNA and protein transfer through pilus and some channels respectively.
Dtr: It is an acronym of the term DNA transfer and replication. Dtr is a gene product engaged in the processing and transfer of plasmid DNA.
The F-pili of the donor cell initiates the process of mating by first binding with the outer membrane protein of the recipient cell. Eventually, a cytoplasmic bridge appears between the F+ and F– cell, which commonly refers to as conjugation tube or pilus. Through this cytoplasmic bridge, one strand of the F-plasmid is nicked by a relaxase enzyme at the oriT site and then moved to the recipient cell from 5’-3’ prime.
The site where the transfer of plasmid DNA occurs commonly refers to as “oriT”. Thus, a pilus formed between the F+ and F– cell facilitates the transfer of F-plasmid DNA. The single stranded DNA moves into the recipient cell will transform into a circular F-plasmid dsDNA. After the completion of conjugation, both the partners will carry F-plasmid DNA.
Mating occurs between High-frequency recombination and F– strains refers as Hfr-F– Conjugation. The strain having F-plasmid integrated with the bacterial chromosome refers as Hfr strains. An Hfr strain will function as a donor and can pass on the chromosomal genes to the F– strain. One strand of the chromosomal DNA from the Hfr strain will move to the recipient cell from the origin of the transfer site.
Unlike conjugation between F+-F– strain, it involves the transfer of full bacterial chromosome and a part of F-plasmid from Hfr donor to the F– strain. In contrast to F+-F– strain conjugation, the only part of F-plasmid is transferred that will not transform F– strain into F+ donor strain.
The replicated donor DNA enters the recipient cell and may degrade into fragments or incorporate with the recipient’s nucleoid via recombination. Hfr-F– Conjugation is important regarding the process of gene mapping, where the relative positions of the gene in a bacterial chromosome can be identified.
Mating occurs between F’ and F– strains refers to as F’-F– Conjugation. F’– strain contains excised F-plasmid integrated with the chromosomal DNA of Hfr strain. F– Strain only contains the bacterial nucleoid and functions as a recipient cell.
This kind of conjugation is virtually identical, where the F’ plasmid enters the F’– strain without being incorporated into the recipient’s nucleoid. Therefore, a recipient cell becomes F’– strain and functions as partially diploid merozygote, by carrying F’– plasmid having two sets of genes.