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This article throws light upon the three transposable elements in eukaryotes.
The three transposable elements in eukaryotes are: (1) Yeast Ty Elements (2) Drosophila Transposons and (3) Controlling Elements in Maize.
Element # 1. Yeast Ty Elements:
The yeast carries about 35 copies of a transposable element called Ty in its haploid genome. These transposons are about 5900 nucleotide-pairs long and are bounded at each end by a DNA segment called the σ sequence, which is ~340bp long. Each σ sequence is oriented in the same direction, forming what are known as direct long terminal repeats of LTRs. Sometimes an LTR becomes detached from a Ty element, creating a so called solo σ. It is thought that these solo σ are generated by recombination between the LTRs of a complete Ty element.
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Ty elements are flanked by five nucleotide-pair direct repeats created by the duplication of DNA at the site of the Ty insertion. These target site duplications do not have a standard sequence, but they tend to contain AT bp. This may indicate that Ty elements preferentially insert into A-T-rich regions of the genome.
The genetic organization of the Ty elements resembles that of the eukaryotic retroviruses. These single stranded RNA viruses synthesize DNA from their RNA after entering a cell. The DNA then inserts into a site in the genome, creating target site duplication. This inserted material has the same overall structure as yeast Ty element-a DNA bounded by LTRs and is called a provirus. Ty elements have only two genes, A and B which are analogous to the gag and pol gene of the retroviruses. These two genes can form virus like particles inside yeast cells.
One hypothesis is that yeast Ty elements are primitive retroviruses, capable of moving from one site to another inside a cell, but not capable of moving between cells. In this regard, is has been shown that the transposition of Ty elements involves an RNA intermediate. After the RNA is synthesized from Ty DNA, a product of the TyB gene uses the RNA to make double stranded DNA. The process is reverse transcription. Then the newly synthesized DNA is inserted somewhere in the genome, creating a new Ty element (12.11). Because of their overall similarity to the retroviruses, yeast Ty elements are sometimes called retro-transposons (12.12).
Element # 2. Drosophila Transposons:
Transposable elements have been discovered in many animals, but some of the best information comes from studies with Drosophila, in which as much as 15% of the DNA is mobile. The largest group of Drosophila transposons comprises the retrovirus like elements, or retro transposons. These elements are 5000 to 15,000 nucleotide pair long and resemble the integrated forms of retro transposon.
P elements-The P element in Drosophila is one of the best examples of exploiting the properties of transposable elements in eukaryotes. This element, shown in figure 12.13, is 2907 bp long and features a 31 bp inverted repeat at each end. DNA sequence analysis of the 2.9 kb element reveals a gene, composed of four exons and three introns, that en-codes transposase. There is a perfect 31 bp inverted repeat at each terminus.
Although the transposase is required for transposition, it can be supplied by a second element. Therefore, P elements with internal deletions can be mobilized and then remain fixed in the new position in the absence of the second element; thus the P element can serve as a convenient marker.
P elements do not utilize an RNA intermediate during transposition and can insert at many different positions in the Drosophila chromosome. The transposition of a P element is controlled by repressors encoded by the element.
P elements have been developed as tools for Drosophila much in the same way that transposons such as Tn 10 have for bacteria. Namely, P elements can be used to create mutations by insertion, to mark the position of genes, and to facilitate the cloning of genes. P elements can be inserted into genes in vivo, and different phenotypes can be selected.
Then, the interrupted gene can be cloned, with the use of P element segments as a probe, a method termed transposon tagging. Primers matching the 31 bp sequence at each end can be used to sequence chromosomal regions adjacent to P element insertion sites.
Element # 3. Controlling Elements in Maize:
McClintock’s Experiments: The Ds Element:
In the 1950s, Barbara McClintock during study of corn kernels found that, rather than being purple or white; they exhibited spots of purple pigment on normally white kernels. She knew that the phenotype was the result of an unstable mutation. From her careful genetic and cytological studies she came to the conclusion that the spotted phenotype was not the result of any conventional kind of mutation but rather was due to a controlling element, which we now know is a transposon.
The explanation for the spotted kernels is that if the corn plant carries a wild type C gene, the kernel will be purple, c (colourless) mutations block purple pigment production, so the kernel is colourless. During kernel development, revertants of the mutation occur, leading to a spot of purple pigment. The genetic nature of the reversion is supported by the fact that descendents of the cell which underwent the reversion also can produce the pigment.
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The earlier in development the reversion occurs, the larger is the purple spot. McClintock determined that the original c (colourless) mutation resulted from a mobile controlling element, a genetic factor called as Ds (Dissociation). This element gets inserted into the C gene. This action of Ds is dependent on the presence of an unlinked gene, Ac (Activator).
Ac is required for transposition of Ds into the gene. Ac can also move the Ds out of the C gene, resulting in the wild type revertant, i.e., a purple spot. McClintock found it impossible to map Ac. In some plants, it mapped to one position; in other plants of the same line, it mapped to different positions.
Moreover, the Ds locus itself was constantly changing position on the chromosome arm, as indicated by the differing phenotypes of the variegated sections of the seeds.
Ac is the autonomous element of the family, and hence mutations caused by Ac are unstable. Ds is the non-autonomous element of the family. Ds mutations are stable if only Ds are present; they are unstable in the presence of an Ac element.
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Ac is 4,563 bp long, with 11bp imperfect terminal inverted repeats (IRs). Ac contains a single transcription unit that comprises most of the element’s length. Ds does not transpose in the absence of Ac and remains as a stable insertion in the chromosome. When an Ac element is present, it activates Ds, causing it to transpose to a new site or to break the chromosome in which it is located.
Ds elements are heterogenous in length and sequence. All Ds elements have the same terminal IRs as Ac elements, and many of these Ds elements have been generated from Ac by deletion of various lengths. Ac exhibits conservative transposition. Ac transposes only during replication (Fig. 12.14).
General Characteristics of Controlling Elements:
Several systems like Ds-Ac have now been found in corn. Each shows similar action, having a target gene that is inactivated, presumably by the insertion of some receptor element into it, and a distant regulator gene that maintains the mutational instability of the locus, presumably through its ability to “unhook” the receptor element from the target locus and return the locus to normal function. The receptor and the regulator are termed controlling elements.
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In the examples considered so far, the unstable allele is said to be non-autonomous: it can revert only in the presence of the regulator. Sometimes, however, a system such as the Ac-Ds system can produce an unstable allele that is autonomous. Such mutants are recognizable because they show Mendelian ratios (such as 3:1 for pigmented to dotted) that apparently are independent of any other element. In fact, such alleles appear to be caused by the insertion of Ac itself into the target gene.
An allele of this type can subsequently be transformed into a non-autonomous allele. In such cases, the non-autonomy seems to result from the spontaneous generation of a Ds element from the inserted Ac element. In other words, Ds is in all likelihood an incomplete version of Ac itself.
