The key difference between LTR and non-LTR retrotransposons is that LTR retrotransposons are a type of retrotransposons that have long terminal repeats in their structure, while non-LTR retrotransposons are a type of retrotransposons that do not have long terminal repeats in their structure.
Retrotransposons (class I transposable elements or transposons via RNA intermediates) are a type of genetic component that can copy and paste themselves into different genomic locations. They perform this by converting RNA back into DNA through a process called reverse transcription using an RNA transposition intermediate. There are two main types of retrotransposons as LTR and non-LTR retrotransposons
CONTENTS
1. Overview and Key Difference
2. What are LTR Retrotransposons
3. What are Non-LTR Retrotransposons
4. Similarities – LTR and Non-LTR Retrotransposons
5. LTR vs Non-LTR Retrotransposons in Tabular Form
6. Summary – LTR vs Non-LTR Retrotransposons
What are LTR Retrotransposons?
LTR retrotransposons are a type of retrotransposons that have long terminal repeats in their structure. Long strands of repetitive DNA can be found at each end of an LTR retrotransposon. These are known as long terminal repeats (LTR). Each LTR is a few hundred base pairs long. LTR retrotransposons are usually over 5 kilobases long. Moreover, between the long terminal repeats, there are genes that can be transcribed equivalent to retrovirus genes gag and pol. These genes overlap so that they encode a protease that normally processes the resulting transcript into functional gene products.
Gag gene of retrotransposons makes a product that associates with other retrotransposons transcripts to form virus-like particles. Pol gene makes products that include enzymes such as reverse transcriptase, integrase, and ribonuclease H domains. The reverse transcriptase enzyme carries out the reverse transcription of retrotransposon DNA. Integrase enzyme integrates retrotransposon DNA into eukaryotic genome DNA. Ribonuclease H domains are helpful in cleaving phosphodiester bonds between RNA intermediates. LTR retrotransposons also encode transcripts with tRNA binding sites so that they can undergo reverse transcription quite easily. Furthermore, since the retrotransposon contains eukaryotic genome information, it can insert copies of itself into other genomic locations within a eukaryotic cell.
What are Non-LTR Retrotransposons?
Non-LTR retrotransposons are a type of retrotransposons that do not have long terminal repeats in their structure. Like LTR retrotransposons, non-LTR retrotransposons also contain genes for reverse transcriptase, RNA binding protein, nuclease, and sometimes ribonuclease H domains. But they lack the long terminal repeats; instead, they have short repeats that can have an inverted order of bases next to each other. The RNA binding proteins bind to the RNA transposition intermediate, and nucleases break phosphodiester bonds between nucleotides in nucleic acids.
Figure 02: LTR and Non-LTR Retrotransposons
Although they are retrotransposons, they cannot carry out reverse transcription using an RNA intermediate in the same way as LTR retrotransposons. This is because non-LTR retrotransposons do not contain sequences that bind tRNA. Hence, a non-LTR retrotransposon transcribed into mRNA and translated into proteins act as reverse transcriptase. Reverse transcriptase makes a DNA copy of the non-LTR retrotransposon’s RNA that can be integrated into the genome at a new site. Furthermore, non-LTR retrotransposons fall into two types: LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements).
What are the Similarities Between LTR and Non-LTR Retrotransposons?
- LTR and non-LTR retrotransposons are the two main types of retrotransposons.
- Both types contain genes for enzymes such as reverse transcriptase, RNA binding protein, nuclease, and ribonuclease H domains.
- They can be found in the human genome.
- Both types are evolutionary important.
What is the Difference Between LTR and Non-LTR Retrotransposons?
LTR retrotransposons have long terminal repeats in their structure, while non-LTR retrotransposons lack long terminal repeats in their structure. Thus, this is the key difference between LTR and non-LTR retrotransposons. Furthermore, LTR retrotransposons have long direct repeats that are just one sequence of bases repeating themselves. On the other hand, non-LTR retrotransposons have short repeats that can have an inverted order of bases next to each other.
The below infographic presents the differences between LTR and non-LTR retrotransposons in tabular form for side by side comparison.
Summary – LTR vs Non-LTR Retrotransposons
LTR and non-LTR retrotransposons are the two main types of retrotransposons. LTR retrotransposons have long terminal repeats in their structure, while non-LTR retrotransposons do not have long terminal repeats in their structure. So, this is the key difference between LTR and non-LTR retrotransposons.
Reference:
1. Havecker, Ericka R, et al. “The Diversity of LTR Retrotransposons – Genome Biology.” BioMed Central, BioMed Central, 18 May 2004.
2. Han, Jeffrey S. “Non-Long Terminal Repeat (Non-LTR) Retrotransposons: Mechanisms, Recent Developments, and Unanswered Questions – Mobile DNA.” BioMed Central, BioMed Central, 12 May 2010.
Image Courtesy:
1. “LTR retrotransposon” By Mariuswalter – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Components of the Human Genome” By Alglascock – Own work (CC BY-SA 3.0) via Commons Wikimedia