Conjugation is an alternate natural method for Horizontal gene Transfer in bacteria. It is defined the process by which one bacterium transfers genetic material to another through direct cell-to-cell contact or by a bridge-like connection between two cells. This bridge is formed with the help of sex pili, it also called as conjugative pili. It looks like hair like appendage on the bacterial cell surface. It is made from pilin proteins.
During conjugation, one bacterium serves as the donor of the genetic material, and the other serves as the recipient. The donor bacterium carries a DNA sequence, F-factor or F plasmid. The F-factor allows the donor to produce a thin, tubelike structure of pilus, which the donor uses to contact the recipient. During conjugation, pilus emerges from the donor cell and get attached to the recipient cell. Its length is around 0.3 to 1 micrometer and its diameter is about 7 nano-meter. The pilus then draws the two bacteria together, at which time the donor bacterium transfers genetic material to the recipient bacterium.
What is the F-factor?
F stands for Fertility factor or F plasmid. Strains containing the F factor are called F+cells and those without are called F- cells. A conjugative plasmid is an example of episome because it can exist as a plasmid or become integrated into the bacterial chromosome. It carries tra operons, which expresses proteins required for pilus formation and DNA transfer. The tra operon consist of two components – Dtr (DNA transfer and replication) & Mpf (Mating pair formation) component.
Dtr – Its abbreviation indicates its function. It is involved in preparing the plasmid for transfer and it carries information for its Replication and Relaxase (an endonuclease enzyme) and information to synthesize proteins required to form Relaxosome complex.
Mpf – The Mating pair formation bring and hold two cells (donor and recipient) together in close proximity and forms the channel via which DNA would be transferred. Hence, it carries information required for pilus formation. Once the channel is formed, Mpf communicate with Dtr component and initiate transfer of DNA.
The steps of conjugation are as follows –
- The F factor is cut at a specific region called the origin of transfer (Ori T) by a protein assembly called the relaxosome which get associate with the DNA strand that needs to be transferred (T-DNA strand) to the recipient cell.
- Accessory protein of the relaxosome are released but the portion of the relaxosome called the relaxase remains attached to the T-DNA.
- This T-DNA relaxase complex is recognized by a coupling factors and transferred to the exporter with the help of transferasome.
- The exporter pumps the T-DNA relaxase complex into the recipient cell.
- Once the entire T-DNA molecule is transferred to the recipient cell, the relaxase joins the ends to make a circular DNA molecule.
- As the T-DNA is transferred to the recipient cell, it is replicated to become double stranded. Double stranded F factor DNA replicates by rolling circle replication using OriV as the origin (V – Vegetative).
- The pilus is released and now both the cells have complete copy of F factor
- Each cell is now an F+ cell and is ready to conjugate other bacteria.
Hfr strain –
A high-frequency recombination cell (Hfr cell) is a bacterium with a conjugative plasmid (for example, the F-factor) integrated into its chromosomal DNA.
The integration of the plasmid into the cell’s chromosome is through homologous recombination.
When F plazmid inaccurately excises from the chromosome of Hfr strain, it can carry a portion of the bacterial chromosome, which later becomes part of the plasmid. Such F plazmid carrying a small segment of bacterial chromosome as F’ Plasmid (F prime).
Conjugation between F+ and F-
This conjugation would result in the successful transfer of the F plasmid and convert the F- cell to F+ Cell and F+ would remain the same. Hence, both the cells become F+.
Conjugation of Hfr and F+ strain –
This conjugation between Hfr nor F+ cells will never occur because none of them can be recipients. Recipient cells in conjugation must be F-.
Conjugation between Hfr and F-
The conjugation between Hfr and F, would result in the formation of F- but it will be in partial diploid state because it will carry a portion of bacterial DNA donated and integrated by F plasmid of Hfr ( plasmid + bacterial genes) during their conjugation.
As you can see in the diagram, the Hfr cell is carrying the F plasmid or F factor in its chromosome. When transfer of DNA starts, it starts from middle portion of F plasmid and try to carry whole Hfr genome to F-, but it never happens and hence only half of F plasmid with few genes of Hfr enters F- cell. This allows transferred Hfr genes to integrate into F- chromosome. The remaining portion of transferred DNA gets degraded and hence, F- remain to be F-.
Conjugation between F’ and F-
F’ is an independent plasmid carrying few bacterial gene. When F’ plasmid conjugates with F-, it transfers its copy to F- and, resulting in the conversion of F- to F’.