Tn3

Tn3 is a bacterial DNA transposon and the prototype of the Tn3 family of transposable elements. It is a classic example of a replicative DNA transposon, moving within and between DNA molecules by forming a cointegrate intermediate. The prototype element carries the beta-lactamase gene bla(Tn3), illustrating how transposons can disseminate antibiotic resistance genes.

The transposon has two essential open reading frames: tnpA, which encodes the transposase responsible for cutting and joining DNA during transposition, and tnpR, which encodes a resolvase that acts at a res site within Tn3 to resolve cointegrates. The ends of Tn3 are flanked by inverted repeats, and a res site within the element serves as the target for the resolvase. Regulatory sequences control the expression of transposition genes, enabling the element to respond to cellular conditions.

Mechanistically, Tn3 transposes via a replicative process that creates a cointegrate between the donor and target DNA. The resolvase then mediates site-specific recombination at the res site to separate the intertwined molecules, resulting in two DNA molecules, each containing a copy of the transposon. This mode of movement allows Tn3 and related elements to spread within a genome and between replicons.

Tn3-family transposons are widespread in Gram-negative bacteria and are commonly found on plasmids and chromosomes. They contribute to genomic plasticity and the horizontal transfer of antibiotic resistance determinants, making them a focus of study in microbial genetics and public health.