Mobile genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In humans, approximately 50% of the genome is thought to be MGEs. MGEs play a distinct role in evolution. Gene duplication events can also happen through the mechanism of MGEs. MGEs can also cause mutations in protein coding regions, which alters the protein functions. They can also rearrange genes in the host genome. One of the examples of MGEs in evolutionary context is that virulence factors and antibiotic resistance genes of MGEs can be transported to share them with neighboring bacteria. Newly acquired genes through this mechanism can increase fitness by gaining new or additional functions. On the other hand, MGEs can also decrease fitness by introducing disease-causing alleles or mutations.
Types
Transposons are DNA sequences that can move locations within a genome, which includes retrotransposons and DNA transposons.
*Retrotransposons are the most widespread class of transposons in mammals. An RNA transcript of MGEs is copied by reverse transcriptase. Then, the DNA sequence can be inserted back to a random location of the genome.
*DNA transposons are a DNA segment that can move to a new location by a "cut-and-paste" strategy.
Bacteriophage elements, like Mu, which integrates randomly into the genome by transduction.
Group I and Group II introns are a product from self-splicing in the host transcripts, and they act as ribozymes that can invade tRNA, rRNA, and protein coding genes in bacteria.
Research examples
systems in bacteria and archaea are adaptive immune systems to protect against deadly consequences from MGEs. Using comparative genomic and phylogenetic analysis, researchers found that CRISPR-Cas variants are associated with distinct types of MGEs such as transposable elements. In addition, CRISPR-Cas controls transposable elements for their propagation. MGEs such as plasmids by a horizontal transmission are generally beneficial to an organism. The ability of transferring plasmids is important in an evolutionary perspective. Tazzyman and Bonhoeffer found that fixation of the transferred plasmids in a new organism is just as important as the ability to transfer them. Beneficial rare and transferable plasmids have a higher fixation probability, whereas deleterious transferable genetic elements have a lower fixation probability to avoid lethality to the host organisms. Transposition by transposable elements is mutagenic. Thus, organisms have evolved to repress the transposition events, and failure to repress the events causes cancers in somatic cells. Cecco et al. found that during early age transcription of retrotransposable elements are minimal in mice, but in advanced age the transcription level increases. This age-dependent expression level of transposable elements is reduced by calorie restriction diet.
The total of all mobile genetic elements in a genome may be referred to as the mobilome. Barbara McClintock was awarded the 1983Nobel Prize in Physiology or Medicine "for her discovery of mobile genetic elements". Mobile genetic elements play a critical role in the spread of virulence factors, such as exotoxins and exoenzymes, among bacteria. Strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed.
