7.12
DNA-only Transposons
DNA-only transposons are called autonomous transposons since they code for the enzyme transposase that is required for the transposition mechanism. In…
Transposons can be found in both prokaryotes and eukaryotes, accounting for 5% of amoeba genome, 44% of the human genome and up to 90% of the maize genome. There are many different types of transposons, but they can be broadly characterized into two distinct classes - one, and two.
Class one transposons, also known as retrotransposons, require transcription of an RNA intermediate into DNA before being inserted into their target.
However, class two transposons - or “DNA only transposons” - remain in DNA form throughout transposition. They also contain a gene encoding a multifunctional enzyme called transposase.
The transposase gene is flanked by terminal inverted repeats which are single-stranded, about 9 to 40 base pairs long, and reverse complements of each other. During cut and paste transposition, the gene expresses two monomers of the transposase enzyme, which bind to the terminal inverted repeats.
When the monomers dimerize, they bring the two inverted repeats together. This creates a stable DNA protein complex called a synaptic complex or a transpososome.
Next the transpososome cleaves the DNA strands on each end of the transposon leaving behind sequences called direct repeats. This releases the transposon from the donor DNA, and it is now free to be transported to a random target.
For insertion, the transposase makes two staggered cuts in the target DNA by cleaving the phosphodiester bonds. The cut strands of the target DNA are pulled apart to create a gap with single-stranded overhangs.
Now, the 3’ OH end of the transposon DNA reacts with the 5’ terminal of the target DNA, leaving gaps between the 5’ ends of the transposon and the 3’ ends of the target.
DNA polymerase uses the 3’ end of the gap as a primer and fills the gap in a process called target site duplication. The newly synthesized DNA and the transposon are joined by DNA ligase.
Such an insertion can have significant possible effects. In some cases, it may turn the target gene on or off by providing novel promoters or insulators encoded by the transposon.
Alternatively, it can also alter the gene function by disrupting normal exon splicing during mRNA generation. For example, this can happen if a transposon containing a new splice site jumps into an existing gene and rewires the transcriptional signals - resulting in the creation of an aberrant mRNA.
Q1: What is the difference between class one and class two transposons?
Class one transposons, called retrotransposons, require transcription into an RNA intermediate before insertion into target DNA. Class two transposons, or DNA-only transposons, remain in DNA form throughout transposition. Both types can move within genomes, but DNA-only transposons use a cut-and-paste mechanism involving the enzyme transposase.
Q2: How do terminal inverted repeats function in DNA-only transposons?
Terminal inverted repeats are short DNA sequences flanking the transposase gene, typically 9 to 40 base pairs long and reverse complements of each other. Transposase monomers bind to these repeats, and when they dimerize, they bring the inverted repeats together to form a stable DNA-protein complex called a transpososome, enabling transposon mobilization.
Q3: What happens during target site duplication in transposon insertion?
During insertion, transposase makes staggered cuts in target DNA, creating single-stranded overhangs. The transposon's 3' OH end joins the target DNA's 5' terminal, leaving gaps. DNA polymerase fills these gaps using the 3' end as a primer, and DNA ligase seals the newly synthesized DNA to the transposon, completing target site duplication.
Q4: How can transposon insertion alter gene expression?
Transposon insertion can alter gene expression by introducing novel promoters or insulators that turn genes on or off. Additionally, if a transposon contains new splice sites, it can disrupt normal exon splicing during mRNA generation, creating aberrant mRNA transcripts and changing the resulting protein structure or function.
Q5: What is exon shuffling and how does it relate to transposition?
Exon shuffling occurs when imperfect excision of transposable elements carries genomic sequences along with the transposon. When inserted at a new location, this positions unrelated exons adjacent to each other, creating new gene structures. This process allows transposition to reorganize non-mobile genetic elements and generate genetic diversity.
Q6: Why are DNA-only transposons considered autonomous elements?
DNA-only transposons are autonomous because they encode transposase, the multifunctional enzyme required for their own transposition mechanism. This self-sufficiency allows them to move independently within the genome without requiring proteins from other sources, making them powerful tools in genome editing and transgenesis applications.
Q7: What is the Sleeping Beauty transposon and how is it used?
The Sleeping Beauty transposon is a synthetic DNA transposon widely used in transgenesis as a gene vehicle to introduce foreign DNA into host organisms. Researchers use it across diverse species ranging from protozoa to small vertebrates like fishes, frogs, and mice to study foreign DNA effects, introduce new traits, or discover novel genes.