The transfer of genetic material between bacteria through cell-to-cell contact is called bacterial conjugation. It is a mechanism of horizontal gene transfer. Bacterial conjugation is often incorrectly regarded as sexual reproduction or mating. It does not involve the fusion of gametes and formation of a zygote. Therefore, it is not actually sexual mating. It is only the transfer of genetic information from a donor cell to a recipient. One of the bacteria acts as donor. It donates conjugative or mobile genetic element in the form conjugative plasmid. Most conjugative plasmids have systems that ensure the recipient cell does not already contain a similar element. The genetic information transferred can be beneficial to the recipient. Some of these are used to transfer antibiotic resistance, or an enzyme that allows it to better digest its medium.
Mechanism of conjugation
I. The prototype for conjugative plasmids is the F-plasmid also
called F-factor. The F-plasmid is an episome. Episome is a plasmid of about 100 kb length that can integrate itself into the bacterial chromosome by genetic recombination. (One kb is one thousand base pairs) It carries its own origin of replication, called oriV. There can only be one copy of the Fplasmid in a bacterium. Such bacterium is called F-positive or
sometimes male bacterium. The bacteria lacking F plasmid are called F-negative or female bacterium.
Fig: Conjugation in bacteria
2. Among other genetic information, the F-plasmid carries. a tra and a trb locus. Both these locus together are about 33 kb long and consist of about 40 genes. The Ira locus has p//in gene and regulatory genes. These genes form sex pill on the cell surface. They also form polymeric proteins. These proteins can attach themselves to the surface of F-negative bacteria and initiate the conjugation. Some proteins coded in the Ira or trb loci. These proteins open a channel between the bacteria.
3. Conjugation is initiated by mating signal. A complex of proteins called the relaxosome creates a nick in one plasmid DNA strand at the origin of transfer, or oriT. In the F-plasmid system the relaxosome consists of proteins Tral, TraY, TraM, and the integrated host factor, IHF.
4. There are two culture of bacteria:
(a) Some culture of cells contains non-integrated F plasmids. These are responsible for the low-frequency of chromosomal gene transfer.
(b) Some Strains of bacteria have integrated F-plasmid. Such strains transfer chromosomal genes very efficiently. So they are called Hfr (high frequency of recombination). A hfr cell is a bacterium with a conjugative plasmid (often F) integrated into its genomic DNA. Unlike a normal F+ cell, hfr strains attempt to transfer their entire DNA through the mating bridges (called pili) to the F- cell. This occurs because the F factor has integrated itself via an insertion point in the bacterial chromosome. The F factor’s has inherent nature to transfer itself over the pilus during bacteria conjugation. Therefore, rest of the bacterial genome is dragged along with it.
5. Mating always occur between F– and F. F-plasmid is integrated into the host genome. Donor chromosomal DNA may be transferred along with plasmid DNA. The chromosome of donor bacteria replicated before this transfer. Therefore, its genes are not lost. The amount of chromosomal DNA that is transferred depends on timing of contact. For common laboratory strains of E. coli the transfer of the entire bacterial chromosome takes about 100 minutes. The transferred DNA can be integrated into the recipient genome via recombination.
6. The newly acquired DNA aligns with the homologous region of cells own chromosome. A crossover changes DNA between
these two. Binary fission of this cell gives rise to new recombinant bacteria. The genes of these bacteria are derived from two different sources.