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At least 19 recordsLinked to original sources
PubMed1992

Transposable elements.

Transposable elements comprise a major fraction of eukaryotic genomes. They are studied both because of their intrinsic biological interest and because they can be exploited as valuable research tools. Many interesting papers dealing with various aspects of the biology of these elements have been published during the past year and a number of new elements have been reported. Four areas in which particularly valuable contributions have been made are the mechanisms of transposition, the regulation of transposition, the use of transposable elements as research tools, and the biological function of transposable elements.

Animals↗
PubMed1986

Molecular biology of transposable elements.

Transposable elements play an essential part in the epidemic spread of antibiotic resistances among bacterial populations. Their ubiquity and their role in the molecular evolution of genomes and organisms has captured the interest of molecular biologists, who investigate the genetic structure and novel mechanisms of recombination underlying transposition. This article, after introducing the general structure of mobile genetic elements and basic mechanistic concepts of transposition in bacteria, uses the well-studied Tn3 family as paradigm for gene organization, gene function and gene regulation. Finally, new applications of transposons in genetic engineering will be described.

DNA Transposable Elements↗
PubMed1986

Evolution of transposable elements.

Transposable elements are at an advantage compared with other DNA sequences. This has led to the generation of transposable systems from a variety of different sources. Once fabricated, these elements can very simply acquire accessory genes, such as those that encode resistance to antibiotics. This can lead to further evolutionary advantages and to their shorter-term importance. It is likely that many new transposable systems will be discovered, owing to the advantages conferred on sequences by transposition, and it is certain that many new transposons that encode varying drug resistance patterns will arise.

Biological Evolution↗
PubMed1992

A maize cryptic Ac-homologous sequence derived from an Activator transposable element does not transpose.

Sequences sharing homology to the transposable element Activator (Ac) are prevalent in the maize genome. A cryptic Ac-like DNA, cAc-11, was isolated from the maize inbred line 4Co63 and sequenced. Cryptic Ac-11 has over 90% homology to known Ac sequences and contains an 11 bp inverted terminal repeat flanked by an 8 bp target site duplication, which are characteristics of Ac and Dissociation (Ds) transposable elements. Unlike the active Ac element, which encodes a transposase, the corresponding sequence in cAc-11 has no significant open reading frame. A 44 bp tandem repeat was found at one end of cAc-11, which might be a result of aberrant transposition. The sequence data suggest that cAc-11 may represent a remnant of an Ac or a Ds element. Sequences homologous to cAc-11 can be detected in many maize inbred lines. In contrast to canonical Ac elements, cAc-11 DNA in the maize genome is hypermethylated and does not transpose even in the presence of an active Ac element.

Base Sequence↗
PubMed1986

The population biology of transposable elements.

A transposable element can be defined as a DNA sequence capable of moving to new sites in the genome. Such DNA sequences have been described in a wide range of organisms. The evolutionary processes affecting transposable elements can thus be divided into two categories: changes in sequence and changes in genomic location. As with other types of evolutionary change, the nature of the evolutionary process will be reflected in the extent and type of genetic variation existing in wild populations. Quantitative models of the evolution of transposable element sequences and positions will be outlined, and related to relevant data. The extent to which models designed to describe obvious transposable elements such as the mobile sequences of Drosophila are also applicable to interspersed repetitive DNAs from other species will be discussed.

Animals↗
PubMed1986

The Mu transposable elements of maize: evidence for transposition and copy number regulation during development.

The Mu transposon of maize exists in a highly mutagenic strain called Robertson's Mutator. Plants of this strain contain 10-50 copies of the Mu element, whereas most maize strains and other plants have none. When Mutator plants are crossed to plants of the inbred line 1S2P, which does not have copies of Mu, the progeny plants have approximately the same number of Mu sequences as did their Mutator parent. Approximately one-half of these copies have segregated from their parent and one-half have arisen by transposition and are integrated into new positions in the genome. This maintenance of copy number can be accounted for by an extremely high rate of transposition of the Mu elements (10-15 transpositions per gamete per generation). When Mutator plants are self-pollinated, the progeny double their Mu copy number in the first generation, but maintain a constant number of Mu sequences with subsequent self-pollinations. Transposition of Mu and the events that lead to copy number maintenance occur very late in the development of the germ cells but before fertilization. A larger version of the Mu element transposes but is not necessary for transposition of the Mu sequences. The progeny of crosses with a Mutator plant occasionally lack Mutator activity; these strains retain copies of the Mu element, but these elements no longer transpose.

Base Sequence↗
PubMed1989

The maize En-1/Spm element transposes in potato.

The maize transposable element En-1 has been introduced into a diploid potato line via transformation with Agrobacterium tumefaciens. The element is transcriptionally active in potato. Numerous En specific RNAs are observed, including a 6 kb transcript characteristic of an active En-1 element in maize. In contrast to maize, where the 6.0 kb transcript is hardly detectable, this transcript is very abundant in the transgenic potato. Transposition of En-1 in the potato clone was analysed by Southern blot hybridization and confirmed by molecular isolation of En-1 excision and integration events. Sequence data of excision and integration sites revealed footprints and target site duplications similar to the ones described for En-1 in maize.

Blotting, Southern↗
PubMed1989

The beta heterochromatic sequences flanking the I elements are themselves defective transposable elements.

Phylogenetic studies suggest that mobile element families are unstable components of the Drosophila genome. Two examples of immobilization of a transposable element family are presented here: as judged by their constant genomic organization among unrelated strains, the F and I element families have been respectively immobilized for a long time in D. simulans and in the reactive D. melanogaster strains (these are the laboratory strains which escaped the recent I invasion of D. melanogaster natural populations). All the elements of these defective families are located in the beta heterochromatic portion of the genome. Moreover, most if not all of the beta heterochromatic sequences into which the defective I elements are embedded are themselves non-mobile members of various nomadic families such as mdg 4, 297, 1731, F and Doc. These results are discussed with special emphasis on the possible nomadic origin of beta heterochromatin components and on the mechanisms of evolutionary turnover of the transposable element families.

Animals↗
PubMed1990

Excision of a Ds-like maize transposable element (Ac delta) in a transient assay in Petunia is enhanced by a truncated coding region of the transposable element Ac.

The excision of a Ds-like transposable element (Ac delta) is mediated in trans by the transposable element Ac or its derivatives in Petunia protoplasts cotransfected with two plasmid DNAs. Excision restores the activity of the beta-glucuronidase (GUS) gene that is otherwise shut off by the presence of Ac delta in its leader sequence. A transient expression assay (histochemical test) is used to detect the beta-glucuronidase activity at the protoplast to detect the beta-glucuronidase activity at the protoplast level. The number of blue-stained protoplasts is a measure of the excision frequency. With Ac delta alone a near-zero background of GUS activity is detected, which is weakly enhanced by the presence, in trans, of either the wild-type Ac or the coding region (ORFa) transcribed from the 2' promoter of Agrobacterium tumefaciens TR-DNA. A strong enhancement is observed when a truncated Ac coding region, also under the control of the 2' promoter, is supplied in trans. The truncated version has ATG10 at codon 103 in frame with ORFa and is preceded by 7 out-of-frame ATGs. The assay is quick and well suited for detection of excision frequencies above the value obtained with the wild-type Ac. The presence of empty donor sites following excision can be demonstrated by PCR amplification and direct sequencing of the appropriate DNA fragment.

Base Sequence↗
PubMed1979

Nomenclature of transposable elements in prokaryotes.

Transposable elements are defined as specific DNA segments that can repeatedly insert into a few or many sites in a genome. They are classified as simple IS elements, more complex Tn transposons and self-replicating episomes. Definitions and nomenclature rules for these three classes of prokaryotic transposable elements are specified.

Cells↗
PubMed1990

Invertrons, a class of structurally and functionally related genetic elements that includes linear DNA plasmids, transposable elements, and genomes of adeno-type viruses.

Invertrons are genetic elements composed of DNA with inverted terminal repeats at both ends, covalently bonded to terminal proteins involved in the initiation of DNA replication at both their 5' termini when they exist in the cytoplasm of their host in free form. They function as viruses, linear DNA plasmids, transposable elements, and sometimes combinations of two of these properties. They differ from retroviruses and related retro-type transposons which have direct repeats on both their genomic ends and exploit RNA intermediates for replication of their DNA. A model for replication and integration of invertrons is presented, as well as a model for transposition of transposable elements.

Adenoviridae↗
PubMed1987

Isolation and characterization of a 1.7-kb transposable element from a mutator line of maize.

We have cloned and sequenced a 1.7-kb Mu element from a Mutator line of maize and compared its structure to Mu1, a 1.4-kb element. With the exception of a 385-bp block of DNA present in the 1.7-kb element, these transposable elements are structurally similar, sharing terminally inverted and internal direct repeated sequences. Derivation of 1.4-kb elements from the 1.7-kb class via deletion of internal sequence is suggested by the finding that a portion of the extra DNA in Mu1.7 is part of a truncated direct repeat sequence in the 1.4-kb element. An abundant poly(A)+ RNA homologous to a portion of this extra DNA is present in several tissues of both Mutator and non-Mutator lines. Analysis of transcripts from an unstable mutant bronze 1 (bz) allele containing a Mu1.7 element inserted in an exon of the gene detects three species of poly(A)+ RNA that hybridize to a Bz1 (Bronze) gene probe: the largest contains the entire Mu1.7 element in the Bz1 gene transcript; another appears to be a spliced, chimeric transcript; the smallest is normal size Bz1 mRNA. The latter is most likely encoded by the normal-size alleles detected by Southern analysis of tissue expressing purple pigment, suggesting that normal gene function is restored by excision of the Mu1.7 element.

Base Sequence↗
PubMed1988

Insertion and excision of Caenorhabditis elegans transposable element Tc1.

The transposable element Tc1 is responsible for most spontaneous mutations that occur in Caenorhabditis elegans variety Bergerac. We investigated the genetic and molecular properties of Tc1 transposition and excision. We show that Tc1 insertion into the unc-54 myosin heavy-chain gene was strongly site specific. The DNA sequences of independent Tc1 insertion sites were similar to each other, and we present a consensus sequence for Tc1 insertion that describes these similarities. We show that Tc1 excision was usually imprecise. Tc1 excision was imprecise in both germ line and somatic cells. Imprecise excision generated novel unc-54 alleles that had amino acid substitutions, amino acid insertions, and, in certain cases, probably altered mRNA splicing. The DNA sequences remaining after Tc1 somatic excision were the same as those remaining after germ line excision, but the frequency of somatic excision was at least 1,000-fold higher than that of germ line excision. The genetic properties of Tc1 excision, combined with the DNA sequences of the resulting unc-54 alleles, demonstrated that excision was dependent on Tc1 transposition functions in both germ line and somatic cells. Somatic excision was not regulated in the same strain-specific manner as germ-line excision was. In a genetic background where Tc1 transposition and excision in the germ line was not detectable, Tc1 excision in the soma still occurred at high frequency.

Animals↗
PubMed1989

Tagging of a maize gene involved in kernel development by an activated Uq transposable element.

A quiescent Uq transposable element has been activated in a maize plant treated with 5-aza-2'-deoxycytidine. This activated Uq cosegregates with a heritable dominant miniature (Mn) kernel phenotype, indicating its physical association with a maize miniature locus (Mn::Uq). The Mn::Uq mutant is dominant in producing a miniature seed phenotype of variable size and in reducing seedling vigor in the early growth stage. Genetic experiments indicate that the Mn::Uq mutant also affects the activity of the male gametophyte, whereby pollen germination is inhibited, thus lacking pollen tube growth resulting in the male nontransmissibility of this mutant. Proof for the Uq element in this mutant is derived by its ability to transactivate the standard a-ruq reporter allele to yield spotted aleurone tissue. However, the Mn::Uq mutant does not transactivate a normally Uq-responsive c-ruq allele, suggesting a structural difference between the two ruq receptors at the A1 and C1 loci. It is anticipated that cloning of the Uq transposable element would facilitate the molecular cloning and characterization of the maize miniature gene.

DNA Transposable Elements↗
PubMed1979

Molecular model for the transposition and replication of bacteriophage Mu and other transposable elements.

A series of molecular events will explain how genetic elements can transpose from one DNA site to another, generate a short oligonucleotide duplication at both ends of the new insertion site, and replicate in the transposition process. These events include the formation of recombinant molecules which have been postulated to be intermediates in the transposition process. The model explains how the replication of bacteriophage Mu is obligatorily associated with movement to new genetic sites. It postulates that all transposable elements replicate in the transposition process so that they remain at their original site while moving to new sites. According to this model, the mechanism of transposition is very different from the insertion and excision of bacteriophage lambda.

Coliphages↗
PubMed1989

Transposable elements and fitness in Drosophila melanogaster.

Transposable elements constitute a significant fraction of the Drosophila melanogaster genome. The five families of moderately repeated transposable elements identified to date occupy dispersed and variable genomic locations, but have relatively constant copy numbers per individual. What effect to these elements have on the fitness of the individuals harboring them? Experimental evidence relating to this question is reviewed. The relevant data fall into two broad categories. The first involves the determination of the distribution of transposable elements in natural populations, by restriction mapping or in situ hybridization, and the comparison of the observed distribution with different theoretical expectations. The second approach is to study directly the effects of new transposable element-induced mutations on fitness. The P family of transposable elements is a particularly efficient mutagen, and the results of experiments in which initially P-free chromosomes are contaminated with P elements are discussed with regard to P-induced fitness mutations.

Animals↗
PubMed1990

The Mu1 transposable element of maize contains two promoter signals recognized by the Escherichia coli RNA polymerase.

The galactokinase (GalK) expression plasmid vector system pKO-1 has been used to screen for promoter elements in the maize transposable element Mu1 that function in Escherichia coli. Two transcriptional start points, named S1 and S2, were identified, which are located in the two direct repeats of the transposable element. This paper demonstrates that sequence elements exist in a plant transposable element which function as prokaryotic promotors.

Base Sequence↗
PubMed1990

Tad, a LINE-like transposable element of Neurospora, can transpose between nuclei in heterokaryons.

The Tad transposon of Neurospora crassa appears to be a LINE-like element with very restricted distribution within the genus Neurospora. When forced heterokaryons were constructed between strains which did and did not contain Tad, the nuclei of the naive nuclear type rapidly acquired Tad elements. The elements acquired by naive nuclei are active, since they can pass Tad to other naive nuclei in subsequent heterokaryons. When heterokaryons are passaged by serial transfer, the load of acquired Tad elements appears to increase, indicating that transposition is continuing in these heterokaryons, even after all of the naive nuclei have acquired Tad. In normal heterokaryons of Neurospora, nuclei do not fuse. An experiment to test for the possibility that Tad promotes nuclear fusion gave negative results. Thus Tad appears to have a cytoplasmic intermediate in its transposition. When heterokaryon incompatible strains were cocultured, there was no indication that Tad elements could be transferred to the naive strain, suggesting that Tad is not a virus. These data are consistent with the transposition of Tad via RNA and cDNA intermediates, as has been postulated to occur with LINE-like elements.

Base Sequence↗