Bacterial conjugation

From Academic Kids

Bacterial conjugation is the transfer of genetic material between bacteria through cell-to-cell contact. (as opposed to transformation or transfection)

Bacterial conjugation is often regarded as the bacterial equivalent of sexual reproduction or mating; however it is not actually sexual as it does not involve the fusing of gametes and the creation of a zygote, 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 plasmid. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element.

These elements are best viewed as genetic parasites on the bacterium, and conjugation as a mechanism evolved by the element to spread itself into new hosts.

The prototype for conjugative plasmids is the F-plasmid, also called the F-factor. The F-plasmid is an episome (a plasmid that can integrate itself into the bacterial chromosome by genetic recombination) of about 100 kb length. (One kb is one thousand base pairs) It carries its own origin of replication, called oriV. There can only be one copy of the F-plasmid in a bacterium (which is then called F-positive), either free or integrated.

Among other genetic information, the F-plasmid carries a tra and a trb locus, which together are about 33 kb long and consist of about 40 genes. The tra locus includes the pilin gene and regulatory genes, which together form pili on the cell surface, polymeric proteins that can attach themselves to the surface of F-negative bacteria and initiate the mating. Though there is some debate on the issue, the pili themselves do not seem to be the structures through which the actual exchange of DNA takes place; rather, some proteins coded in the tra or trb loci seem to open a channel between the bacteria.

When conjugation is initiated, via a 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 TraI, TraY, TraM, and the integrated host factor, IHF. The transferred, or T-strand, is unwound from the duplex and transferred into the recipient bacterium in a 5'-terminus to 3'-terminus direction. The remaining strand is replicated, either independent of conjugative action (vegetative replication, beginning at the oriV) or in concert with conjugation (conjugative replication similar to the rolling circle replication of lambda phage).

If the F-plasmid becomes integrated into the host genome, donor chromosomal DNA may be transferred along with plasmid DNA. The amount of chromosomal DNA that is 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 the low-frequency of chromosomal gene transfer by such cultures. 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 are called Hfr (high frequency of recombination). The E. coli genome was originally mapped by interupted 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.

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