text: The Tn7 transposon is a mobile genetic element found in many prokaryotes such as Escherichia coli (E. coli), and was first discovered as a DNA sequence in bacterial chromosomes and naturally occurring plasmids that encoded resistance to the antibiotics trimethoprim and streptomycin. Specifically classified as a transposable element (transposon), the sequence can duplicate and move itself within a genome by utilizing a self-encoded recombinase enzyme called a transposase, resulting in effects such as creating or reversing mutations and changing genome size. The Tn7 transposon has developed two mechanisms to promote its propagation among prokaryotes. Like many other bacterial transposons, Tn7 transposes at low-frequency and inserts into many different sites with little to no site-selectivity. Through this first pathway, Tn7 is preferentially directed into conjugable plasmids, which can be replicated and distributed between bacteria. However, Tn7 is unique in that it also transposes at high-frequency into a single specific site in bacterial chromosomes called attTn7. This specific sequence is an essential and highly conserved gene found in many strains of bacteria. However, the recombination is not deleterious to the host bacterium as Tn7 actually transposes downstream of the gene after recognizing it, resulting in a safe way to propagate the transposon without killing the host. This highly evolved and sophisticated target-site selection pathway suggests this pathway evolved to promote coexistence between the transposon and it host, as well as Tn7's successful transmission into future generations of bacterium.
  The Tn7 transposon is 14 kb long and codes for five enzymes. The ends of the DNA sequence consists of two segments that the Tn7 transposase interacts with during recombination. The left segment (Tn7-L) is 150 bp long and the right sequence (Tn7-R) is 90 bp long. Both ends of the transposon contain a series of 22 bp binding sites that the Tn7 transposase recognizes and binds to. Within the transposon are five discrete genes encoding for proteins that make up the transposition machinery. In addition, the transposon contains an integron, a DNA segment containing several cassettes of genes encoding for antibiotic-resistance.
  The Tn7 transposon codes for five proteins: TnsA, TnsB, TnsC, TnsD, and TnsE. TnsA and TnsB interact together to form the Tn7 transposase enzyme TnsAB. The enzyme specifically recognizes and binds to the ends of the DNA sequence of the transposon, and excises it by introducing double-stranded DNA breaks to each end. The excised sequence is then inserted to another target DNA site. Much like other characterized transposons, the mechanism for Tn7 transposition involves cleavage of the 3 ends from the donating DNA by the TnsA protein of the TnsAB transposase. However, Tn7 is also uniquely cleaved near the 5 ends, about 5 bp from the 5 end towards the Tn7 transposon, by the TnsB protein of TnsAB. After the insertion of the transposon into the target DNA site, the 3 ends are covalently linked to the target DNA, but the 5 bp gaps are still present at the 5 ends. As a result, repair of these gaps leads to a further 5 bp duplication at the target site. The TnsC protein interacts with the transposase enzyme and the target DNA to promote the excision and insertion processes. The ability of TnsC to activate the transposase depends on its interaction with a target DNA along with its appropriate targeting protein, TnsD or TnsE. The TnsD and TnsE proteins are alternative target selectors that are also DNA binding activators that promote excision and insertion of Tn7.  Their ability to interact with a particular target DNA is key to the target-site selection of Tn7. The proteins TnsA, TnsB, and TnsC thus form the core machinery of Tn7: TnsA and TnsB interact together to form the transposase, while TnsC functions as a regulator of the transposases activity, communicating between the transposase and TnsD and TnsE.  When the TnsE protein interacts with the TnsABC core machinery, Tn7 preferentially directs insertions into conjugable plasmids. When the TnsD protein interacts with TnsABC, Tn7 preferentially directs insertions downstream into a single essential and highly conserved site in the bacterial chromosome. This site, attTn7, is specifically recognized by TnsD.