Bacterial conjugation was discovered by Nobel Prize winners Joshua Lederberg and Edward Tatum. They showed that the bacterium Escherichia coli entered a sexual phase during which it could share genetic information.
Bacterial conjugation is often incorrectly regarded as the equivalent of sexual reproduction, since it involves the exchange of genetic material. During conjugation the donor cell provides a conjugative or mobilizable genetic element that is most often a plasmid or transposon. Most conjugative plasmids have systems ensuring that the recipient cell does not already contain a similar element.
The genetic information transferred is often beneficial to the recipient. Benefits may include antibiotic resistance, xenobiotic tolerance or the ability to use new metabolites. Such beneficial plasmids may be considered bacterial endosymbionts. Other elements, however, may be viewed as bacterial parasites and conjugation as a mechanism evolved by them to allow for their spread.
Mechanism[change | change source]
The basic conjugative plasmid is the F-plasmid, or F-factor. The F-plasmid is an episome (a plasmid that can integrate itself into the bacterial chromosome) with a length of about 100,000 base pairs.
There can only be one copy of the F-plasmid in a given bacterium, either free or integrated, and bacteria that possess a copy are called F-positive or F-plus (denoted F+). Cells that lack F plasmids are called F-negative or F-minus (F-) and can function as recipient cells.
Inter-kingdom transfer[change | change source]
For example, the tumor-inducing (Ti) plasmid of Agrobacterium and the root-tumor inducing (Ri) plasmid of A. rhizogenes contain genes that are capable of transferring to plant cells. These genes turn plant cells into factories producing chemicals used by the bacteria for nitrogen and energy. Infected cells form crown galls or root tumours, respectively. The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.
Genetic engineering[change | change source]
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, and isolated mammalian mitochondria.
Conjugation has advantages over other forms of genetic transfer. In plant engineering, Agrobacterium-like conjugation complements other standard vehicles such as tobacco mosaic virus (TMV). While TMV 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.
References[change | change source]
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- Griffiths AJF; et al. (1999). An introduction to genetic analysis (7th ed.). San Francisco: W.H. Freeman. ISBN 0-7167-3520-2. Explicit use of et al. in:
- Lederberg, Joshua, & E.L. Tatum 1946. Gene recombination in E. coli. Nature 158 558, October 19.
- Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 60–4. ISBN 0838585299.CS1 maint: extra text: authors list (link)
- Russi; et al. (2008). "Molecular machinery for DNA translocation in bacterial conjugation". Plasmids: current research and future trends. Caister Academic Press. ISBN 978-1-904455-35-6. Explicit use of et al. in:
- Holmes RK, Jobling MG (1996). Genetics: exchange of genetic information. in: Baron's Medical Microbiology (Baron S et al., eds) (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1.
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- Yoon YG, Koob MD (2005). "Transformation of isolated mammalian mitochondria by bacterial conjugation". Nucleic Acids Res. 33 (16): e139. doi:10.1093/nar/gni140. PMC 1201378. PMID 16157861. http://nar.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=16157861.