Horizontal gene transfer in prokaryotes
Horizontal gene transfer is common among bacteria, even very distantly-related ones. This process is thought to be a significant cause of increased drug resistance; when one bacterial cell acquires resistance, it can quickly transfer the resistance genes to many species. Enteric bacteria appear to exchange genetic material with each other within the gut in which they live. There are three common mechanisms for horizontal gene transfer:
- Bacterial transformation , the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign genetic material, generally DNA(see this animation with audio explanation). This process is relatively common in certain naturally transformable bacteria, such as the soil bacterium Bacillus subtilis, but less common in eukaryotes. Transformation is often used as a biotechnology to insert novel genes into bacteria for scientific research experiments, or for industrial or medical applications. See also molecular biology and biotechnology.
- Transduction, the process in which bacterial DNA is moved from one bacterium to another by a bacterial virus (a bacteriophage, commonly called a phage).
- Bacterial conjugation, a process in which a living bacterial cell transfers genetic material through direct cell-to-cell contact (see the animation conjugation in Escherichia coli).
Major discoveries on horizontal gene transfer in prokaryotes
1928. Discovery of gene transfer in bacteria started in 1928 when Frederick Griffith found Diplococcus pneumoniae bacteria could inherit characteristics (namely the "Smooth" phenotype affecting the bacterial cell coat (capsule) from non-living extracts of other bacteria, and this change was termed transformation. Oswald T. Avery at the Rockefeller Institute in 1944 showed that the transforming material of Diplococcus was DNA.
1946. Joshua Lederberg and Edward Tatum in a series of very carefully designed experiments managed very low-frequency of genetic transfer between Escherichia coli K-12 bacteria when they were in contact with one-another [1], and this process was later found out by the recognized plasmid, fertility factor F.
1951 Joshua and Ester Lederberg , together with Zinder and Lively reported another case of gene transfer that did not need cell-to-cell contact. In this case the bacteria were Salmonella and it was later that a bacterial virus (bacteriophage) was carring the genes transduction.
Transmissible plasmids in microorganisms
1950-1958. Conjugation in E. coli is shown to be a one way (F+ donor to F- recipient), process, and not two-way cell fusion, encoded by a DNA containing factor F 250,000 base pairs in size that converts recipients to the donor state, and which can move infectiously through a recipient bacterial population[2]. F factor was found to be able to move by conjugation to different species such as Salmonella.
1958. The existence of several genetic structures that can insert within bacterial chomosomes, based on observation of the bacteriophage lambda and fertility factor in 1958 F lead F. Jacob and E. L. Wollman [3] to coin the term episome for DNA elements that have alternate modes of existence within the cell, either in the chromosome, or as autonomously replicating stuctures. Subsequent study of these phenomenon revealed numerous occurences of mobile DNA in a wide range of organisms [4](such as the presence of insertion sequence (IS) "jumping genes" that allow F plamid insertion in the chromosome) and widespread horizontal gene transfer involving by bacteriophage, plasmids and mobile DNA in general.
1959. Tomoichiro Akiba and Kunitaro Ochia discover mobile antibiotic resistance genes in bacteria [5], and the horizontal transfer is later shown to mediated by plasmids that inject DNA promiscuously into other cells [6].
1964. Brinton establishes that F factor containing bacteria have pili (fimbrae) fiber-like appendages that are involved making contact with recipients to they can transfer DNA. Similar pili are specified by some R-plasmids.
1960s. R-plasmids are established to be promiscuously transferred among different enteric bacteria. Typically plasmids can be transferred easily among the different species of common gut Gram negative bacteria. Transmissible plasmids are common in stool bacteria, and typically 10-20% of gut E. coli of healthy people (1960-1970) have R-plamids and are antibiotic resistant[7]. Bacteria are discovered to produce colicins, lethal compounds that kill other bacteria which are specified by transmissable plasmids similar to F. Examples are ColE, ColV plasmids.
It is also fully realised that F-factor is not unique, but a special case of a very general phenomenon in bacteria of transmissible and mobile DNA, including R-plasmids, colicin determining (Col) plasmids, and prophages. Interactions between them were discovered, for instance R-plasmids were found which interfered with F-fertility. Plasmids were also discovered that carried bacterial virulence genes, Hemolysin (Hly), and surface antigens (K88). Plasmids are shown to be closed circular DNA molecules which are much smaller than the circular bacterial genome.
Insertion sequences (IS), transposons (Tn)and Mu
1968. James Shapiro discovers that spontaneously occuring insertions of large inserts of extra DNA can causes mutationss in the galactose genes of the bacterium Escherichia coli [8]. This discovery ultimately led to the discovery of mobile insertion sequences (IS). Their similarity to Barbara McClintock's maize transposons was immedidiately relised when IS were found through DNA heteroduplex electron microscopy and other methods to be natural residents of the E. coli chromosome, and frequently components of natural plamids [9]. F-factor carries more than one IS element, and they (Tn1000) are implicated in enabling bacterial chromosomes to take part in F-mediated conjugation.
1963-1995. Mutator phage Mu of E. coli which inserts at mumerous different sites in the E. coli chromosomes as a prophage, and replicates vegetatively by repeated transposition, provides a tool for elucidation of the mechanisms of transposition in great detail [10] [11]which usually, being a rare event, is difficult to analyse biochemically.
1974. The first transposition of a gene for ampicillin resistance from one R-factor to another is fully characterised [12]. The Apr transposon is named Tn3. Other transposons are Tn5 (kanamycin resistance with IS50 at each termini) , and Tn10 ( tetracyline resistance).
Backbones and islands found in genomes
1990s. Availability of many genome squences reveals bacteria such as E. coli have genomes which consist of a "backbone" of conserved genes and "islands" of changeable genes that are foreign DNA derived inserts (such as inserted prophages. Lawrence and Ochman (1998) [13] calculate there is constant turnover of horizontally transferred DNA in these islands in E. coli at the rate of ~16 thousand base-pairs per million years.
References
- ↑ Lederberg, J. and Tatum, E. L. (1946). Novel genotypes in mixed cultures of biochemical mutants of bacteria. Cold Spring Habor Symposia of Quantitative Biology. 11, p113.
- ↑ Hayes, W. (1970) The Genetics of Bacteria and their Viruses. 2nd Edition, Blackwell.
- ↑ Jacob, F. and Wollman, E. L. (1958) Les episomes, elements genetiques ajoutes. C. R. Acad, Sci. Paris, 247, p154.
- ↑ Berg, D. E. and Howe, M. M. (Eds.)(1989). Mobile DNA. American Society for Microbiology. Washington, D.C.
- ↑ Ochia, K. Yamanaka, T. Kimura, K. and Sawada, O. (1959). Inheritance of drug resistance (and its transfer) between Shigella strains and between Shigella and E. coli strains. Nihon Iji Shimpo 1861: p34 (In Japanese)
- ↑ S. Falcow (1975)Infectious Multiple Drug Resistance. Pion Press, London.
- ↑ Falcow, S. (1975). Infectious multiple Drug Resistance, Pion.
- ↑ Shapiro, J. (1969) Mutations caused by the insertion of genetic material into the galactose operon of Escherichia coli. J. Molec. Biol. 40, p93-109.
- ↑ Galas, D. J. and Chandler, M. (1989). Bacterial insertion sequences. In Mobile DNA Berg, D E and Howe, E, eds. (Washington, DC: American Society for Microbiology), pp. 109-162.
- ↑ Mizuuchi, K. (1992) Transpositional recombination: mechanistic insights from studies of Mu and other elements. Annu. Rev. Biochem. 61, 1011-1051.
- ↑ Pato, M. L. (1989) Bacteriophage Mu. In Mobile DNA Berg, D E and Howe, E, eds. (Washington, DC: American Society for Microbiology), pp. 23-52.
- ↑ Hedges, R. W. and Jacob, A. F. Transposition of ampicillin resistance from RP4 to other replicons. Mol. Gen. Genetics 132 pages 31-40.
- ↑ Lawrence, J.G. and Ochman, H. (1998) Molecular archeology of the Escherichia coli genome. PNAS 95, pages 9413-9417.
Further reading
- Snyder, L. and Champness, W. (2003) Molecular Genetics of Bacteria, 2nd Edition, ASM Press Washington DCISBN 1-55581-204-X
- Hayes, W. (1970) The Genetics of Bacteria and their Viruses. 2nd Edition, Blackwell.