Bacterial conjugation is the 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 exchange of genetic information. In order to perform conjugation, one of the bacteria has to carry an F-plasmid, the other one must not.
The F-plasmid (also called F-factor) is an episome (a plasmid that can integrate itself into the bacterial chromosome by genetic recombination) of about 100 kb (kilo base pairs) length. 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 controlling 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. 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.
The transfer of DNA always runs from the F-positive towards the F-negative bacterium.
- If the F-plasmid is free, a nick is produced in one of the circular DNA strands of the plasmid. Then, the single-stranded DNA of the nicked strand is inserted into the recipiane bacterium (5'-end first). The now single-stranded plasmid is filled up with a complementary strand by a rolling cycle mechanism.
- If the F-plasmid is integrated, the transferred DNA consists of the F-plasmid sequence plus an amount of chomosomal DNA from the donor bacterium. The amount of chromosomal DNA that is transferred depends on how long the bacteria hold contact; the transfer of the whole bacterial chromosome would take about 100 minutes. The transferred DNA can be integrated into the recipients DNA by recombination. Bacteria with an integrated F-plasmid do recombine unusually often, which is why they are also called Hfr-(high frequency recombination-)strains.
See also