Everything about Bacterial Conjugation totally explained
Bacterial conjugation' is the transfer of genetic material between bacteria through direct cell-to-cell contact. Discovered in 1946 by
Joshua Lederberg and
Edward Tatum, conjugation is a mechanism of
horizontal gene transfer—as are
transformation and
transduction—although these mechanisms don't involve cell-to-cell contact.
Bacterial conjugation is often incorrectly regarded as the
bacterial equivalent of
sexual reproduction or
mating. It isn't actually sexual, as it doesn't involve the fusing of
gametes and the creation of a
zygote, nor is there equal exchange of genetic material. It is merely the transfer of genetic information from a donor cell to a recipient. In order to perform conjugation, one of the bacteria, the
donor, must play host to a conjugative or mobilizable genetic element, most often a conjugative or mobilizable
plasmid or
transposon. Most conjugative plasmids have systems ensuring that the
recipient cell doesn't already contain a similar element.
The genetic information transferred is often beneficial to the recipient cell. Benefits may include
antibiotic resistance, other
xenobiotic tolerance, or the ability to utilize a new
metabolite. Such beneficial plasmids may be considered bacterial
endosymbionts. Some conjugative elements may also be viewed as genetic
parasites on the bacterium, and conjugation as a mechanism was evolved by the mobile element to spread itself into new hosts.
Mechanism
The
prototype for conjugative plasmids is the
F-plasmid, also called the F-factor.
If the F-plasmid becomes integrated into the host genome, donor chromosomal DNA may be transferred along with plasmid DNA.
The certain amount of chromosomal DNA that's transferred depends on how long the bacteria remain in 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.
A culture of cells containing non-integrated F plasmids usually contains a few that have accidentally become integrated, and these are responsible for those low-frequency chromosomal gene transfers which do occur in such cultures. Some strains of bacteria with an integrated F-plasmid can be isolated and grown in pure culture. Because such strains transfer chromosomal genes very efficiently, they're called
Hfr (
high
frequency of
recombination). The
E. coli genome was originally mapped by interrupted mating experiments, in which various Hfr cells in the process of conjugation were sheared from recipients after less than 100 minutes (initially using a
Waring blender) and investigating which genes were transferred.
Inter-kingdom transfer
The
nitrogen fixing Rhizobia are an interesting case, wherein conjugative elements naturally engage in inter-
kingdom conjugation. Such elements as the
Agrobacterium Ti or Ri plasmids contain elements that can transfer to plant cells. Transferred genes enter the plant cell nucleus and effectively transform the plant cells into factories for the production of
opines, which the bacteria use as carbon and energy sources. Infected plant cells form
crown gall or
root tumors. The Ti and Ri plasmids are thus
endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.
The Ti and Ri plasmids are themselves conjugative. Ti and Ri transfer between bacteria uses an independent system (the
tra, or transfer, operon) from that for inter-kingdom transfer (the
vir, or
virulence, operon). Such transfer creates virulent strains from previously avirulent
Agrobacteria.
See also
Further Information
Get more info on 'Bacterial Conjugation'.
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