7.14: LTR Retrotransposons
LTR retrotransposons are class I transposable elements with long terminal repeats flanking an internal coding region. These elements are less abundant in mammals compared to other class I transposable elements. About 8 percent of human genomic DNA comprises LTR retrotransposons. Some of the common examples of LTR retrotransposons are Ty elements in yeast and Copia elements in Drosophila.
The internal coding region of LTR retrotransposons and their mechanism of transposition closely resembles a retroviral genome. They can encode enzymes required for their mobilization as well as synthesize structural proteins to form a virus-like particle. But most LTR retrotransposons lack the genes to synthesize a viral envelope. Hence, in contrast to the retroviruses that can form infectious virions and move horizontally from one cell to another, LTR retrotransposons are only allowed to move from one locus to another within the genome of a single cell.
However, some LTR retroelements have been found to have an extra ORF in the same position as the “env” gene found in retrovirus genomes, for example, gypsy elements in Drosophila. These elements can encode for three putative proteins, one of which resembles a retroviral envelope protein that results in an infectious form of the element.
In humans, the most common LTR retrotransposons are called endogenous retroviruses or ERVs. These ERVs are products of ancestral exogenous viral infections in the germline cells, which lead to their vertical transmission in humans. However, because of co-evolution spanning millions of years, these ERVs now play an important role in human physiology, including maintaining certain regulatory networks.