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.
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.
The nitrogen fixing "Rhizobia" are an interesting case of inter-kingdom conjugation.
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.
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.
Plasmid
A plasmid is a DNA molecule that is separate from the chromosomal DNA and that can replicate (copy itself) independently.
The term "plasmid" was first introduced by the American molecular biologist Joshua Lederberg in 1952.
Plasmids are double stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms (e.g., the "2-micrometre-ring" in "Saccharomyces cerevisiae").
Plasmid size varies from 1 to over 1,000 kilobase pairs (kbp). The number of identical plasmids within a single cell can range from one to thousands. Plasmids are often associated with conjugation, a mechanism of horizontal gene transfer.
Plasmids are transferable genetic elements, or "replicons", capable of independent self copying in a suitable host. Plasmids can be found in all three major domains, Archaea, Bacteria and Eukarya. Similar to viruses, plasmids are not considered a form of "life" as it is currently defined. Unlike viruses, plasmids are "naked" DNA and do not encode genes necessary to encase the genetic material for transfer to a new host.
Plasmid host-to-host transfer requires direct, mechanical transfer by conjugation or changes in host gene expression allowing the intentional uptake of the genetic element by transformation.
Microbial transformation with plasmid DNA is neither parasitic nor symbiotic in nature, since each implies the presence of an independent species living in a commensal or detrimental state with the host organism. Rather, plasmids provide a mechanism for horizontal gene transfer within a population of microbes and may provide a selective advantage under a given environmental state.
Plasmids may carry genes that provide resistance to naturally occurring antibiotics in a competitive environmental niche, or alternatively the proteins produced may act as toxins under similar circumstances. Plasmids also can provide bacteria with an ability to fix elemental nitrogen or to degrade difficult organic compounds. This may provide an advantage under conditions of nutrient deprivation.
Horizontal gene transfer
Horizontal gene transfer (HGT) (or "Lateral gene transfer") is any process in which an organism gets genetic material from another organism without being the offspring of that organism.
By contrast, "vertical transfer" occurs when an organism gets genetic material from its ancestor, e.g., its parent or a species from which it has evolved.
Most thinking in genetics has focused on vertical transfer, but there is a growing awareness that horizontal gene transfer is a significant phenomenon. Amongst single-celled organisms it may be the dominant form of genetic transfer. Artificial horizontal gene transfer is a form of genetic engineering.
There are several mechanisms for horizontal gene transfer:
Horizontal gene transfer was first described in Japan in a 1959 publication that demonstrated the transfer of antibiotic resistance between different species of bacteria.
In the mid-1980s, Syvanen suggested that lateral gene transfer not only had biological significance, but was involved in shaping evolutionary history from the beginning of life on Earth.
"Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes". The phenomenon appears to have had some significance for unicellular eukaryotes as well. As Bapteste et al. observe, "additional evidence suggests that gene transfer might also be an important evolutionary mechanism in protist evolution".
There is some evidence that even higher plants and animals have been affected. Richardson and Palmer (2007) state: "Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryotes. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remains unclear".
Transduction (genetics)
Transduction is the process by which DNA is transferred from one bacterium to another by a virus.
Joshua Lederberg and his graduate student Norton Zinder showed in 1952 that bacteriophages could transfer genetic information between bacteria in "Salmonella". This process explained how bacteria of different species could gain resistance to the same antibiotic very quickly.
Transduction also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. This is a common tool used by molecular biologists to introduce a foreign gene into a host cell's genome.
When bacteriophages infect a bacterial cell, their normal mode of reproduction is to harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous complete viral particles, including the viral DNA or RNA and the protein coat. Errors in this process can result in the virus carrying over DNA from one bacterium to another.
Ferdinand de Lesseps
Ferdinand Marie, Vicomte () de Lesseps, GCSI (19 November 1805 – 7 December 1894) was the French developer of the Suez Canal, which joined the Mediterranean and Red Seas in 1869. He also tried to build a Panama Canal during the 1880s, but malaria and yellow fever was so bad that he gave up and the Panama Canal Company went bankrupt. A different canal was later built by the United States and completed in 1914.
Ferdinand de Lesseps was born at Versailles, Yvelines, in 1805. His first years were spent in Italy, where his father was a diplomat.
In 1828 de Lesseps was sent as an assistant vice-consul to Tunis, where his father was consul-general. He aided the escape of Youssouff, pursued by the soldiers of the Bey, of whom he was one of the officers, for violation of the seraglio law.