Transposons, also called “jumping genes”, are DNA fragments that can move within or between genomes in a process called transposition. In bacteria, transposons are involved in the transmission of antibiotic resistance and virulence genes. Bacterial Tn7 elements are among best-studied and most widespread DNA transposons. Tn7 mobility is mediated by five-element encoded proteins. Translocation occurs via cut-and-paste mechanism executed by a heteromeric transposase TnsA+TnsB, which is recruited to the target by the AAA+ ATPase TnsC which interacts with one of the two target selectors, TnsD or TnsE. TnsD directs the element to the conserved chromosomal attTn7 site, while TnsE allows the transposition to the conjugal plasmids. The CRISPR-associated transposon (CAST) elements that use widely recognized CRISPR-Cas systems for RNA-guided DNA transposition are all related to Tn7 and encode TnsB-like transposases. They may provide more precise tool for next generation gene editing.
The scientists from the Laboratory of Protein Structure collaborated with Joe Peters group from Cornell University to study the structure of mechanism of prototypic E. coli Tn7 TnsB. They used cryoelectron microscopy (cryo-EM) to determine the structure of a complex of TnsB with double-stranded DNA substrates corresponding to the right end of the transposon at 2.7 Å resolution. The structure shows that when TnsB interacts with repeating binding sites, it adopts a beads-on-a-string architecture, where the DNA-binding and catalytic domains are arranged in a tiled and intertwined fashion. TnsB forms few base-specific contacts with DNA that leads to a binding preference rather than strict specificity. The formation of an array of TnsB molecules bound to the multiple weakly conserved sites at the appropriate spacing converts this preference into specific end recognition. The scientists proposed a model of the TnsB strand-transfer complex that helps to understand the mechanism of how Tn7 TnsB can interpret subtle differences in the spacing of diverged binding sites and explain central features of Tn7 transposition systems.
Kaczmarska Z, Czarnocki-Cieciura M, Górecka-Minakowska KM, Wingo RJ, Jackiewicz J, Zajko W, Poznański JT, Rawski M, Grant T, Peters JE, Nowotny M. Structural basis of transposon end recognition explains central features of Tn7 transposition systems. Mol Cell, 2022 Jul 21;82(14):2618-2632.e7. doi: 10.1016/j.molcel.2022.05.005