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Tn5 transposase loops DNA in the absence of Tn5 transposon end sequences

Transposases mediate transposition first by binding specific DNA end sequences that define a transposable element and then by organizing protein and DNA into a highly structured and stable nucleoprotein ‘synaptic’ complex. Synaptic complex assembly is a central checkpoint in many transposition mechanisms. The Tn5 synaptic complex contains two Tn5 transposase subunits and two Tn5 transposon end sequences, exhibits extensive protein–end sequence DNA contacts and is the node of a DNA loop. Using single‐molecule and bulk biochemical approaches, we found that Tn5 transposase assembles a stable nucleoprotein complex in the absence of Tn5 transposon end sequences. Surprisingly, this end sequence‐independent complex has structural similarities to the synaptic complex. This complex is the node of a DNA loop; transposase dimerization and DNA specificity mutants affect its assembly; and it likely has the same number of proteins and DNA molecules as the synaptic complex. Furthermore, our results indicate that Tn5 transposase preferentially binds and loops a subset of non‐Tn5 end sequences. Assembly of end sequence‐independent nucleoprotein complexes likely plays a role in the in vivo downregulation of transposition and the cis‐transposition bias of many bacterial transposases.