What Is a Mariner Transposon and How Does It Work?

The genomes of living organisms contain mobile DNA sequences known as transposable elements, or “jumping genes,” which can change their position within a genome to alter genetic information and influence evolution. Among the diverse types of transposable elements, mariner transposons stand out due to their widespread presence across the animal and plant kingdoms. Originally discovered in the fruit fly Drosophila mauritiana, mariner elements have an exceptionally broad host range. Their ability to move between vastly different species has made them a subject of intense study, revealing much about genome dynamics and providing powerful tools for genetic research.

Defining Features of Mariner Transposons

Mariner transposons are classified as Class II, or DNA, transposons, which means they move throughout the genome as a segment of DNA rather than being copied through an RNA intermediate. A typical mariner element is simple and compact, usually ranging from 1,300 to 2,400 base pairs in length. This DNA contains two primary components: a single gene, known as an open reading frame (ORF), and flanking sequences called inverted terminal repeats (ITRs). The ITRs are short, mirrored sequences of DNA that mark the ends of the transposon.

The central open reading frame encodes a protein called transposase, the enzyme that drives the transposon’s movement. This basic architecture of ITRs flanking a transposase-coding gene is a defining characteristic shared across mariner subfamilies, such as cecropia and irritans.

The Mariner Transposition Mechanism

The process by which mariner elements move is known as “cut-and-paste” transposition, involving the physical excision and reinsertion of the DNA segment. The cycle begins when the cell’s machinery produces the transposase enzyme from the transposon’s gene. Once produced, transposase proteins locate the mariner element by recognizing and binding to the specific DNA sequences of the inverted terminal repeats at each end.

After binding, the transposase molecules form a stable complex with the DNA, known as a synaptic complex or transpososome. This structure brings the two ends of the transposon together, preparing it for removal. The transposase then makes precise cuts in the DNA backbone, freeing the transposon from its original position. This action creates a double-strand break at the donor site, which the host cell must then repair.

The liberated transposon-transposase complex then searches for a new location. Mariner transposons show a preference for inserting into a simple two-base-pair sequence, a TA dinucleotide. Upon finding a target site, the transposase integrates the transposon into the new location, creating small gaps that are filled by the cell’s repair machinery. A signature of this event is the duplication of the TA target site on either side of the newly inserted transposon. The original double-strand break is also repaired, though this can be imprecise and cause small mutations.

Distribution and Evolutionary Significance

One of the most remarkable features of mariner transposons is their exceptionally wide distribution across different branches of life. They have been identified in a vast array of organisms, including insects, fungi, plants, and vertebrates. In mammals, including humans, these copies, such as Hsmar1 and Hsmar2, are inactive “fossil” elements that have accumulated mutations and can no longer move on their own.

The broad and patchy distribution of mariner elements is strong evidence that they have spread through horizontal gene transfer (HGT), the movement of genetic material between different species. This means that mariner transposons have jumped between unrelated organisms, such as from an insect to a plant. Potential vectors like viruses or parasites that move between hosts are considered likely candidates for this transfer.

By moving within and between genomes, mariner elements have had a significant impact on host evolution. Their insertion into or near genes can act as a mutagenic force, disrupting gene function or altering how genes are regulated, which can lead to new traits. Over millions of years, the cumulative activity of these transposons contributes to large-scale genome rearrangements and drives genetic diversity.

Applications of Mariner Elements in Research

Scientists have harnessed the mobility of mariner transposons, turning them into versatile tools for molecular biology and genetics. The mariner system is highly adaptable for laboratory use because it is self-contained and can function in a wide variety of organisms without needing host-specific factors. This makes it valuable for genetic studies in non-model organisms where other genetic tools may be inefficient.

A primary application of mariner is for insertional mutagenesis. Researchers can introduce an engineered mariner transposon into an organism to generate random mutations. When the transposon “jumps” into a gene, it can disrupt that gene’s function. By observing the resulting change in the organism’s traits, scientists can deduce the role of the affected gene. This approach is useful for discovering gene functions on a genome-wide scale.

Mariner elements are also widely used for transgenesis, the process of introducing foreign DNA into an organism’s genome. A piece of DNA of interest can be placed between the mariner ITRs, and when the transposase is supplied, the entire segment is integrated into the host’s chromosomes. This method is effective for creating genetically modified organisms for research. To improve the efficiency of these techniques, researchers have engineered hyperactive versions of mariner transposases, such as derivatives of Mos1 and Himar1.