Tn3family

The Tn3 family is a group of bacterial transposable elements that rearrange DNA by a replicative, copy-and-paste mechanism. Prototype members, such as Tn3, carry two essential genes: tnpA, which encodes the transposase that drives DNA strand transfer, and tnpR, which encodes a site-specific resolvase that helps finalize transposition. Many elements in this family also harbor antibiotic resistance determinants, which has made them notable in studies of antimicrobial resistance spread.

Structure and organization commonly include terminal inverted repeats (TIRs) flanking the element and a central res (resolution) site. The res site is a multi-subunit target recognized by the TnpR resolvase; recombination at this site resolves cointegrates that form during replicative transposition. This coordinated action of transposition and resolution distinguishes the Tn3 family from non-replicative transposons and underpins their propagation as autonomous mobile elements.

Mechanistically, transposition begins with limited DNA processing by TnpA at the element ends, followed by replication-driven transfer that creates a cointegrate between donor and target DNA. TnpR then acts at the res site to separate the cointegrate, yielding two intact molecules and leaving a new copy of the transposon integrated into the recipient. The process can be influenced by host factors and the regulatory interplay between TnpA and TnpR, contributing to variable transposition rates in different environments.

Distribution and impact: Tn3 family elements are found across diverse Gram-negative and some Gram-positive bacteria, located on plasmids or chromosomes. By mobilizing and sometimes carrying resistance genes, they contribute to horizontal gene transfer and the evolution of bacterial genomes.