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Tn10 is a transposable element, which is a sequence of DNA that is capable of mediating its own movement from one position in the DNA of the host organism to another. There are a number of different transposition mechanisms in nature, but Tn10 uses the non-replicative cut-and-paste mechanism. [1] The transposase protein recognizes the ends of the element and cuts it from the original locus. The protein-DNA complex then diffuses away from the donor site until random collisions brings it in contact with a new target site, where it is integrated. To accomplish this reaction the 50 kDa transposase protein must break four DNA strands to free the transposon from the donor site, and perform two strand exchange reactions to integrate the element at the target site. This leaves two strands unjoined at the target site, but the host DNA repair proteins take care of this. The target site selection is essentially random, but there is a preference for the sequence 5'-GCTNAGC-3'. The 6-9 base pairs that flank the sequence also influence selection of the insertion site. [2]

Cut-and-paste transposition does not cause an increase in the number of transposons per se: there is one copy at the start and one copy at the end. If this was the end of the matter the transposon would perish by genetic drift and the loss of copies owing to the occasional failure to achieve successful integration at the target site. However, the transposon has a mechanism to favor transposition immediately after a replication fork passes through, leaving a hemimethylated copy of Tn10 on each sister chromosome. Since transposition is favored when Tn10 is hemimethylated, the transposon on one sister chromosome can hop somewhere onto the other chromosome so that two copies of the transposon end up on one chromosome. [3]

Tn10 has a composite structure and it is composed of a pair of insertion sequence elements (IS10) flanking five genes. Only one of the IS10 elements encodes a functional transposase. [4] Since the ends of the IS10 element contain the transposase recognition sites, Tn10 has a total of four such sites. If the transposase binds the two recognition sites flanking an IS10 element, the IS10 element undergoes transposition independently of the larger composite structure. If the transposase binds the two outermost recognition sites, the whole composite Tn10 structure undergoes transposition.

Two of the five genes encoded by the central portion of Tn10, tetA and tetR , confer resistance to the antibiotic tetracycline. The TetA protein is an efflux pump. It has served as a model system for such proteins and has accumulated hundreds of publications indexed in PubMed. The functions of the other three genes, jemA, jemB and jemC, are unknown but they may implicated in heavy metal resistance or oxidative stress. [5]

The Tn10/IS10 transposase is closely related to another composite transposon, Tn5/IS50, which harbors a gene for kanamycin resistance in the unique (i.e. non-repeated) central region of the transposon.

The Tn10 transposon is often used in genetics to transfer and select-for genes of interest from one organism into the chromosome of another.

The mechanism of Tn10 transposition has served as a model system and the archetype for the cut-and-paste mechanisms. However, the transposase is difficult to work with in vitro and the Tn5 transposase was the first to be crystallized. Tn10 was one of the great work-horses of bacterial genetics for many years during which it served as a useful tool. A commercial kit for Tn5 transposition is commercially available and is extensively used in post-genomic technologies.

Two comprehensive reviews of Tn10 biology are available as chapters in the book Mobile DNA and Mobile DNA II. [6] [7]

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Transposase is an enzyme that binds to the end of a transposon and catalyses its movement to another part of the genome by a cut and paste mechanism or a replicative transposition mechanism. The word "transposase" was first coined by the individuals who cloned the enzyme required for transposition of the Tn3 transposon. The existence of transposons was postulated in the late 1940s by Barbara McClintock, who was studying the inheritance of maize, but the actual molecular basis for transposition was described by later groups. McClintock discovered that pieces of the chromosomes changed their position, jumping from one chromosome to another. The repositioning of these transposons allowed other genes for pigment to be expressed. Transposition in maize causes changes in color; however, in other organisms, such as bacteria, it can cause antibiotic resistance. Transposition is also important in creating genetic diversity within species and adaptability to changing living conditions. During the course of human evolution, as much as 40% of the human genome has moved around via methods such as transposition of transposons.

P elements are transposable elements that were discovered in Drosophila as the causative agents of genetic traits called hybrid dysgenesis. The transposon is responsible for the P trait of the P element and it is found only in wild flies. They are also found in many other eukaryotes.

Insertion sequence

Insertion element is a short DNA sequence that acts as a simple transposable element. Insertion sequences have two major characteristics: they are small relative to other transposable elements and only code for proteins implicated in the transposition activity. These proteins are usually the transposase which catalyses the enzymatic reaction allowing the IS to move, and also one regulatory protein which either stimulates or inhibits the transposition activity. The coding region in an insertion sequence is usually flanked by inverted repeats. For example, the well-known IS911 is flanked by two 36bp inverted repeat extremities and the coding region has two genes partially overlapping orfA and orfAB, coding the transposase (OrfAB) and a regulatory protein (OrfA). A particular insertion sequence may be named according to the form ISn, where n is a number ; this is not the only naming scheme used, however. Although insertion sequences are usually discussed in the context of prokaryotic genomes, certain eukaryotic DNA sequences belonging to the family of Tc1/mariner transposable elements may be considered to be, insertion sequences.

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A composite transposon is similar in function to simple transposons and insertion sequence (IS) elements in that it has protein coding DNA segments flanked by inverted, repeated sequences that can be recognized by transposase enzymes. A composite transposon, however, is flanked by two separate IS elements which may or may not be exact replicas. Instead of each IS element moving separately, the entire length of DNA spanning from one IS element to the other is transposed as one complete unit. Composite transposons will also often carry one or more genes conferring antibiotic resistance.

The Tn3 transposon is a 4957 base pair mobile genetic element, found in prokaryotes. It encodes three proteins:

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Knockout rat

A knockout rat is a genetically engineered rat with a single gene turned off through a targeted mutation used for academic and pharmaceutical research. Knockout rats can mimic human diseases and are important tools for studying gene function and for drug discovery and development. The production of knockout rats was not economically or technically feasible until 2008.

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Transposons are semi-parasitic DNA sequences which can replicate and spread through the host's genome. They can be harnessed as a genetic tool for analysis of gene and protein function. The use of transposons is well-developed in Drosophila and in Thale cress and bacteria such as Escherichia coli.

The Sleeping Beauty transposon system is a synthetic DNA transposon designed to introduce precisely defined DNA sequences into the chromosomes of vertebrate animals for the purposes of introducing new traits and to discover new genes and their functions. It is a Tc1/mariner-type system, with the transposase resurrected from multiple inactive fish sequences.

The PiggyBac (PB) transposon is a mobile genetic element that efficiently transposes between vectors and chromosomes via a "cut and paste" mechanism. During transposition, the PB transposase recognizes transposon-specific inverted terminal repeat sequences (ITRs) located on both ends of the transposon vector and efficiently moves the contents from the original sites and integrates them into TTAA chromosomal sites. The powerful activity of the PiggyBac transposon system enables genes of interest between the two ITRs in the PB vector to be easily mobilized into target genomes. The TTAA-specific transposon piggyBac is rapidly becoming a highly useful transposon for genetic engineering of a wide variety of species, particularly insects. They were discovered in 1989 by Malcolm Fraser at the University of Notre Dame.

Ac/Ds transposable controlling elements was the first transposable element system recognized in maize. The Ac Activator element is autonomous, whereas the Ds Dissociation element requires an Activator element to transpose. Ac was initially discovered as enabling a Ds element to break chromosomes. Both Ac and Ds can also insert into genes, causing mutants that may revert to normal on excision of the element. The phenotypic consequence of Ac/Ds transposable element includes mosaic colors in kernels and leaves in maize.

Transposable elements are short strands of repetitive DNA that can self-replicate and translocate within the eukaryotic genome, and are generally perceived as parasitic in nature. Their transcription can lead to the production of dsRNAs, which resemble retroviruses transcripts. While most host cellular RNA has a singular, unpaired sense strand, dsRNA possesses sense and anti-sense transcripts paired together, and this difference in structure allows an host organism to detect dsRNA production, and thereby the presence of transposons. Plants lack distinct divisions between somatic cells and reproductive cells, and also have, generally, larger genomes than animals, making them an intriguing case-study kingdom to be used in attempting to better understand the epigenetics function of transposable elements.

Conservative transposition

Transposition is the process by which a specific genetic sequence, known as a transposon, is moved from one location of the genome to another. Simple, or conservative transposition, is a non-replicative mode of transposition. That is, in conservative transposition the transposon is completely removed from the genome and reintegrated into a new, non-homologous locus, the same genetic sequence is conserved throughout the entire process. The site in which the transposon is reintegrated into the genome is called the target site. A target site can be in the same chromosome as the transposon or within a different chromosome. Conservative transposition uses the "cut-and-paste" mechanism driven by the catalytic activity of the enzyme transposase. Transposase acts like DNA scissors; it is an enzyme that cuts through double-stranded DNA to remove the transposon, then transfers and pastes it into a target site.

DNA transposons are DNA sequences, sometimes referred to "jumping genes", that can move and integrate to different locations within the genome. They are class II transposable elements (TEs) that move through a DNA intermediate, as opposed to class I TEs, retrotransposons, that move through an RNA intermediate. DNA transposons can move in the DNA of an organism via a single-or double-stranded DNA intermediate. DNA transposons have been found in both prokaryotic and eukaryotic organisms. They can make up a significant portion of an organism's genome, particularly in eukaryotes. In prokaryotes, TE's can facilitate the horizontal transfer of antibiotic resistance or other genes associated with virulence. After replicating and propagating in a host, all transposon copies become inactivated and are lost unless the transposon passes to a genome by starting a new life cycle with horizontal transfer. It is important to note that DNA transposons do not randomly insert themselves into the genome, but rather show preference for specific sites.

Tc1/mariner is a class and superfamily of interspersed repeats DNA transposons. The elements of this class are found in all animals, including humans. They can also be found in protists and bacteria.


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