Bacterial Conjugation and Plasmids

Bacterial conjugation

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http://en.wikipedia.org/wiki/Bacterial_conjugation

Bacterial conjugation is the transfer of genetic material between bacteria through cell-to-cell contact. It is a mechanism of horizontal gene transfer, as are transformation and transduction, although these mechanisms do not involve cell contact. This is a rare occurrence in bacteria,.

Bacterial conjugation is often incorrectly regarded as the bacterial equivalent of sexual reproduction or mating. 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 movable genetic element, most often a conjugative plasmid. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element.

The genetic information transferred can be beneficial to the recipient, such as in conferring antibiotic resistance, or an enzyme that allows it to better digest its medium. However these elements can also be viewed as genetic parasites on the bacterium, and conjugation as a mechanism evolved by the element to spread itself into new hosts.

Mechanism

Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipent cell. 4- Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.

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 conjugation.

There is some debate on the actual involvement of pili, 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, The transferred, , 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) 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 certain 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. 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 are 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.