What are Transposons?

Transposons, being a class of mobile genetic elements, are, in the very strictest sense, DNA sequences that are capable of “moving around” a genome. The general structure of a simple transposable element consists of a central region of DNA flanked by inverted repeats, or the same sequence that runs in opposite directions on opposite sides of the DNA strands. The central region of some more complex transposable elements is sometimes flanked by short, direct repeats, which are then flanked by the inverted repeats. These mobile elements can be anywhere from a few hundred base pairs long to a few thousand; they are very abundant throughout the genomes of all living things, with some scientists claiming that they make up at least 10% of the genomes of eukaryotes. They were first discovered in maize, or Indian corn, by Dr. Barbara McClintock in the 1950s; she received the Nobel Prize in Physiology or Medicine for her work, in 1983.

There is some debate as to how many different “groups” of transposons there are, so in this article we are going to make things simple. We will look at two main types of transposable elements: DNA transposons and retrotransposons.

Retrotransposons, sometimes referred to as Class I mobile genetic elements, operate by first transcribing a copy of themselves into RNA and then using the enzyme reverse transcriptase to transcribe themselves back into DNA; this reverse transcribed DNA is then inserted into a new location. The reverse transcriptase is often encoded by the element itself. Many readers will notice a striking similarity with retroviruses, of which HIV is the most well known. Indeed, retroviruses work in a surprisingly similar manner to retrotransposons; the main difference between the two elements is that retroviruses also encode a variety of other viral proteins, some which allow the virus to survive outside the host. Retrotransposons cannot survive outside the host genome.

Within the category of retrotransposons, researchers have found some elements that are similar to retroviruses, called viral retrotransposons, which possess long terminal repeats, or LTRs, as well as elements called LINES, which do not have LTRs. There are some retrotransposons, furthermore, that do not themselves encode for reverse transcriptase; it is thought that these elements take advantage of the enzyme encoded by other retrotransposons.

Most DNA transposons, sometimes referred to as Class II mobile genetic elements, actually use two enzymes, transposase and integrase, to move from one location in the genome to another; they are literally “cut” out of their location and “pasted” into a new one. This type of transposition does not involve RNA. The enzymes may recognize specific sequences for insertion, though many types of enzyme can actually bind to virtually any sequence, allowing the element to be inserted anywhere. At the site where the transposon is to be inserted, the DNA is cut in what’s called a “staggered” manner to create “sticky ends”; what this means is that nucleotides are left without corresponding paired bases. Once the transposon is inserted into the target site, a DNA polymerase adds the missing nucleotides. This filling in of the gaps leads to duplication of short DNA sequences flanking the transposon and this has been hypothesized as a mechanism behind gene duplication. Finally, some DNA transposons actually will replicate themselves, with the copy then being inserted into the target site.

Just as occurs with retrotransposons, there have been cases observed in which DNA transposable elements have lost the enzymes necessary for transposition. They continue to be able to move thanks to the presence of enzymes encoded by other mobile elements. Besides the enzymes important for transposition, some bacterial transposons actually carry genes that are unrelated to movement and that confer an advantage; these are the genes responsible for the rapid spread of antibiotic resistance among bacteria.