Insertion sequences are short, mobile DNA segments found in various genomes, capable of relocating to different positions within the genetic material. Their mobility influences gene function and contributes to an organism’s genetic diversity. Understanding these elements helps illuminate how genomes change over time.
What Are Insertion Sequences?
Insertion sequences, or IS elements, are the simplest form of transposable elements. These genetic units are small, ranging from about 700 to 2,500 base pairs. Their structure is straightforward, consisting primarily of a gene that codes for an enzyme called transposase.
The transposase gene is flanked by short, inverted repeat sequences at each end. These inverted repeats, usually between 10 to 40 base pairs long, are recognized by the transposase enzyme and are necessary for the element’s movement. While predominantly found in bacterial and archaeal genomes, insertion sequences can also reside on plasmids (small, circular DNA molecules separate from the main chromosome) and bacteriophages (viruses that infect bacteria).
How Do Insertion Sequences Move?
Insertion sequences primarily move through a “cut-and-paste” transposition mechanism. This process begins with the transposase enzyme, encoded by the IS element, recognizing the inverted repeats at its ends. The transposase then excises the IS element from its original DNA location.
After excision, the transposase guides the IS element to a new, often random, target site within the genome. The enzyme then inserts the IS element into this new position. This insertion duplicates a short segment of target site DNA, resulting in direct repeats that flank the newly inserted IS element. This movement occurs directly at the DNA level and does not involve an RNA intermediate.
Their Role in Genetic Change
The movement of insertion sequences contributes significantly to genetic change within an organism’s genome. When an IS element inserts into an existing gene, it can disrupt the gene’s sequence, often leading to its inactivation and a change in traits. This disruption is a common source of mutations.
Insertion sequences also play a role in gene regulation. Their insertion into regions upstream of a gene, such as promoter or regulatory sequences, can alter gene expression. This can either activate a gene or reduce its activity, depending on the insertion’s location and orientation. Beyond individual genes, IS elements contribute to broader genome evolution by facilitating large-scale rearrangements like deletions, inversions, and gene duplications.
A key consequence of IS element activity is their involvement in the spread of antibiotic resistance genes among bacteria. These elements can mobilize larger genetic segments carrying resistance genes, effectively transferring these traits between different bacterial strains or species. This ability makes insertion sequences important contributors to the challenge of antibiotic resistance.
Insertion Sequences Versus Other Mobile DNA
Insertion sequences are distinct from other mobile genetic elements, such as composite transposons. While both can move within a genome, IS elements are considered simpler. They typically carry only the genetic information necessary for their own transposition, primarily the gene encoding the transposase enzyme.
In contrast, composite transposons are larger and more complex. They consist of two IS elements positioned at their ends, flanking a central DNA segment that often contains additional genes. These accessory genes confer traits like antibiotic resistance. Other transposable elements, such as retrotransposons, utilize an RNA intermediate during transposition, distinguishing them from the DNA-based “cut-and-paste” mechanism of insertion sequences.