Mobile genetic elements
Mobile genetic elements (MGEs), sometimes called selfish genetic elements,[1] 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.[2] 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. These mechanisms can also rearrange genes in the host genome generating variation. These mechanism can increase fitness by gaining new or additional functions. An example of MGEs in evolutionary context are that virulence factors and antibiotic resistance genes of MGEs can be transported to share genetic code with neighboring bacteria. However, MGEs can also decrease fitness by introducing disease-causing alleles or mutations.[3] The set of MGEs in an organism is called a mobilome, which is composed of a large number of plasmids, transposons and viruses.[4]
Research examples[edit]
CRISPR-Cas 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 CRISPR-associated transposons, CRISPR-Cas controls transposable elements for their propagation.[28]
MGEs such as plasmids by a horizontal transmission are generally beneficial to an organism. The ability of transferring plasmids (sharing) is important in an evolutionary perspective. Tazzyman and Bonhoeffer found that fixation (receiving) of the transferred plasmids in a new organism is just as important as the ability to transfer them.[29] Beneficial rare and transferable plasmids have a higher fixation probability, whereas deleterious transferable genetic elements have a lower fixation probability because they are lethal to the host organisms.
One type of MGEs, namely the Integrative Conjugative Elements (ICEs) are central to horizontal gene transfer shaping the genomes of prokaryotes enabling rapid acquisition of novel adaptive traits.[30][31]
As a representative example of ICEs, the ICEBs1 is well-characterized for its role in the global DNA damage SOS response of Bacillus subtilis[32] and also its potential link to the radiation and desiccation resistance of Bacillus pumilus SAFR-032 spores,[33] isolated from spacecraft cleanroom facilities.[34][35][36]
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.[37] This age-dependent expression level of transposable elements is reduced by calorie restriction diet. Replication of transposable elements often results in repeated sequences being added into the genome. These sequences are often non coding but can interfere with coding sequences of DNA. Though mutagenetic by nature, transposons increase the genome of an organism that they transpose into. More research should be conducted into how these elements may serve as a rapid adaptation tool employed by organisms to generate variability. Many transposition elements are dormant or require activation. should also be noted that current values for coding sequences of DNA would be higher if transposition elements that code for their own transposition machinery were considered as coding sequences.
Some others researched examples include Mavericks,[38][39][40] Starships[41][40] and Space invaders (or SPINs)[42][43]
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Diseases[edit]
The consequence of mobile genetic elements can alter the transcriptional patterns, which frequently leads to genetic disorders such as immune disorders, breast cancer, multiple sclerosis, and amyotrophic lateral sclerosis. In humans, stress can lead to transactional activation of MGEs such as endogenous retroviruses, and this activation has been linked to neurodegeneration.[44]
Other notes[edit]
The total of all mobile genetic elements in a genome may be referred to as the mobilome.
Barbara McClintock was awarded the 1983 Nobel Prize in Physiology or Medicine "for her discovery of mobile genetic elements" (transposable elements).[45]
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.[46]
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