Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. It is a mechanism of horizontal gene transfer as are transformation and transduction although these two other mechanisms do not involve cell-to-cell contact. Bacterial conjugation is often regarded as the bacterial equivalent of sexual reproduction or mating since it involves the exchange of genetic material.
However, it is not sexual reproduction, since no exchange of gamete occurs, and indeed no generation of a new organism: During conjugation the donor cell provides a conjugative or mobilizable genetic element that is most often a plasmid or transposon. The genetic information transferred is often beneficial to the recipient.
Benefits may include antibiotic resistancexenobiotic tolerance or the ability to use new metabolites. Other elements, however, may be viewed as bacterial parasites and conjugation as a mechanism evolved by them to allow for their spread.
The process was discovered in by Joshua Lederberg and Edward Tatum. The F-plasmid is an episome a plasmid that can integrate itself into the Bacteria reproduce asexually by conjugation of dormir chromosome by homologous recombination with a length of about kb.
It carries its own origin of replicationthe oriVand an origin of transfer, or oriT. Among other genetic information, the F-plasmid carries a tra and trb locuswhich 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.
Though there is some debate on the exact mechanism of conjugation it seems that the pili are not the structures through which DNA exchange occurs. has been shown in experiments where the pilus are allowed to make contact, but then are denatured with SDS and yet DNA transformation still proceeds. Several proteins coded for in the tra or trb locus seem to open a channel between the bacteria and it is thought that the traD enzyme, located at the base of the pilus, initiates membrane fusion.
When conjugation is initiated by a signal the relaxase enzyme creates a nick in one of the strands of the conjugative plasmid at the oriT. Relaxase may work alone or in a complex of over a dozen proteins Bacteria reproduce asexually by conjugation of dormir collectively as a relaxosome. The nicked strand, or T-strandis then unwound from the unbroken strand and transferred to the recipient cell 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.
Conjugative replication may require a second nick before successful transfer can occur.
A recent report claims to have inhibited conjugation with chemicals that mimic an intermediate step of this second nicking event. If the F-plasmid that is transferred has previously been integrated into the donor's genome producing an Hfr strain ["High Frequency of Recombination"] some of the donor's chromosomal DNA may also be transferred with the plasmid DNA. In common laboratory strains of E.
The transferred DNA can then be integrated into the recipient genome via homologous recombination.
A cell culture that contains in its population cells with non-integrated F-plasmids usually also contains a few cells that have accidentally integrated their plasmids. It is these cells that are responsible for the low-frequency chromosomal gene transfers that 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 are called Hfr h igh f requency of r ecombination.
The genes that were transferred were then investigated. Since integration of the F-plasmid into the E.
Conjugation in Mycobacteria smegmatislike conjugation in E. The lengths of the donor segments vary widely, but have an average length of Since a mean of 13 tracts are transferred, the average total of transferred DNA per genome is kb.
Bacteria related to the nitrogen fixing Rhizobia are an interesting case of inter- kingdom conjugation. The expression of these genes effectively transforms the plant cells into opine -producing factories. Opines are used by the bacteria as sources of nitrogen and energy.
Infected 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 can also be transferred between bacteria using a system the traor transfer, operon that is different and independent of the system used for inter-kingdom transfer the viror virulenceoperon.
Such transfers create virulent strains from previously avirulent strains. Conjugation is a convenient means for transferring genetic material to a variety of targets. In laboratories, successful transfers have been reported from bacteria to yeast,  plants, mammalian cells,   diatoms  and isolated mammalian mitochondria.
In plant engineering, Agrobacterium -like conjugation complements other standard vehicles such as tobacco mosaic virus TMV. While Bacteria reproduce asexually by conjugation of dormir is capable of infecting many plant families these are primarily herbaceous dicots.
Agrobacterium -like conjugation is also primarily used for dicots, but monocot recipients are not uncommon. From Wikipedia, the free encyclopedia.
Baron S, et al. Univ of Texas Medical Branch. An Introduction to genetic analysis 7th ed.
Sherris Medical Microbiology 4th ed. Exchange of Genetic Information. Baron's Medical Microbiology 4th ed. Transfection Chromosomal crossover Gene conversion Fusion gene Horizontal gene transfer Sister chromatid exchange Transposon. Antigenic shift Reassortment Viral shift.