Structural basis of transposon end recognition explains central features of Tn7 transposition systems

Tn7 is a bacterial transposon with relatives containing element-encoded CRISPR-Cas systems mediating RNA-guided transposon insertion. Here, we present the 2.7 angstrom cryoelectron microscopy structure of proto-typic Tn7 transposase TnsB interacting with the transposon end DNA. When TnsB interacts across 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. The DNA-binding domains form few base-specific contacts lead-ing to a binding preference that requires multiple weakly conserved sites at the appropriate spacing to achieve DNA sequence specificity. TnsB binding imparts differences in the global structure of the protein-bound DNA ends dictated by the spacing or overlap of binding sites explaining functional differences in the left and right ends of the element. We propose a model of the strand-transfer complex in which the ter-minal TnsB molecule is rearranged so that its catalytic domain is in a position conducive to transposition.

Automated summary

The study presents the cryoelectron microscopy structure of Tn7 transposase TnsB, revealing its beads-on-a-string architecture when interacting with the transposon end DNA. The spacing or overlap of TnsB binding sites explains the functional differences in the left and right ends of the element.