It was the first extensively studied method of gene transfer. Conjugation occurs between two living cells, involves cell-to-cell contact, and requires mobilization of either a plasmid or a chromosome of donor bacterial cells.
Conjugative plasmid transfers themselves between bacteria, which had led to the spread of antibiotic resistance among pathogenic bacteria.
Before talking about the mechanism of conjugation let’s first revisit some of the key information related to the plasmid.
- Plasmids are small, circular pieces of DNA that are separate and replicate independently from the bacterial chromosome.
- Plasmid carries genes associated with specialized functions such as drug resistance.
- Plasmids may encode genes that mediate their transfer from one organism to another but not all plasmids are capable of conjugative transfer
- Plasmids that can transmit from one cell to another cell independently are called self-transmissible plasmids. Non-transmissible plasmids are not able to go through independent transmission.
The F plasmid also called the ‘fertility factor’ confers donor characteristics (sex pilus) to bacterial cells.
- F+ strains: Bacteria that have F plasmid are referred to as F+ or male/donor.
- F- strains: Those that do not have F plasmid are called F- or female/recipient.
- Hfr strains: Arise from F+ strains when the F plasmid is incorporated into the bacterial chromosomes at one of several possible sites.
- F’ (prime) strains: F plasmid with a fragment of chromosomes with it.
Mechanism of plasmid mobilization by conjugation
- Conjugative plasmids initiate gene transfer by altering the cell surface to allow contact between the plasmid-containing donor cell (F+ or male) and a plasmid lacking recipients (F- or female).
- Sex pilus originates from the donor and establishes a conjugative bridge (temporary cytoplasmic bridge) that serves as the conduit for DNA transfer from donor to recipient bacterial cell
- Intercellular contact established.
- A copy of DNA from the donor cell (F+ cell) transfers to the recipient cell (F- cell).
- A complementary DNA strand is synthesized in both the donor cell and the recipient cell.
- As the recipient cell now contains F plasmid and behaves as a donor cell.
If F- cells and F+ cells are mixed in a culture, the entire population quickly becomes F+. Conjugation is a conservative type of gene transfer as the donor retains a copy of plasmid DNA. It also suggests that plasmid replication occurs during conjugation.
Examples of Bacteria undergoing Conjugation
- Crown gall tumor in plants by Agrobacterium tumefaciens where the TDNA part of the Ti (tumor inducing) plasmid of the bacterium transfers into the plant cell by conjugation.
- Likewise, transfer of R plasmids in between Shigella species by conjugation leads to antibiotic-resistant Shigella-mediated dysentery.
- Snustad, D., & Simmons, M. (2012). Principle of Genetics (6th ed., pp. 175-179). Hoboken, NJ: Wiley.
- Watson, J., Baker, T., Bell, S., Gann, A., Levine, M., & Losick, R. (2004). Molecular Biology of the Gene (5th ed., pp. 655-658). San Francisco: Pearson Education, Inc.,.