Transposons and Their Mechanisms of Transposition in Prokaryotes
Have you ever wondered how bacteria become resistant to different antibiotics? The answer is through mobile genetic elements called transposons, where the drug-resistant gene lies. Prokaryotic transposons consist of two types: Composite and Simple transposons.
In a composite transposon, a variety of genes reside between two similar insertion sequence elements that are positioned in the opposite direction forming an inverted repeat sequence. In order for the entire transposon to become mobile, it needs a transposase encoded by one of the insertion sequences elements. Transposase is an enzyme required for the movement of insertion sequence elements from one chromosome to another.
On the other hand, a simple transposon consists of genes that are surrounded by inverted repeat sequences similar to the composite transposon. However, the difference is that these sequences are short and do not encode the transposase enzyme needed to move. Simple transposons encode their own transposase in the region between the inverted repeat sequences as well as carrying the genes. Differences aside, composite and simple transposons are referred to as just transposons. A transposon can move from a plasmid to a bacterial chromosome or from one plasmid to another. The mechanisms by which they transpose however, differs.
Transposase is the key enzyme responsible for the mobility of a transposon. More specifically, it plays a role in the excision from the original location and insertion into the new location. Prokaryotes employ two methods of transposition: replicative and conservative.
In the replicative pathway, another copy of the transposable element is created during transposition. The result is one copy at the new site and one copy at the old site. During transposition, the donor and recipient plasmids are fused together to form a double plasmid which is encoded by the Tn3 transposase. Tn3 makes single strand cuts at both ends of Tn3 and staggered cuts at the target sequence joining the free ends together to form a cointegate. Then the transposable element is replicated during fusion and the cointegrate resolves into two smaller circles, leaving a copy of the transposable element in each plasmid. This results in one copy still at the original location and one copy at a new position in the genome.
In contrast, the conservative pathway, as the name suggests does not involve replication. Rather, the transposable element is excised out from the chromosome and integrated into the new site. In other words, the transposable element is cut and pasted. Like replicative transposition, transposase makes cuts at the ends of the transposon. However, unlike replicative transposition, the transposable element is cut out of the donor site. Then staggered cuts are made at the target site where the transposable element is inserted.