Formation of the active Hermes transpososome is driven by asymmetric DNA binding of BED domains

The cut-and-paste Hermes DNA transposase stands out among the transposases that have been biochemically or structurally characterized so far. Many transposases function as dimers, but the Hermes transposase forms a tetramer of dimers to achieve its active form in vivo. Intriguingly, the transposition complex, or transpososome, relies on only one dimer to perform the enzymatic reactions necessary to the mobilization of its transposon. Our investigation combining biochemical and structural approaches shows that the Hermes octamer extensively interacts with its transposon left-end (LE) engaging the BED domains of three Hermes protomers belonging to three dimers. By contrast, the right-end (RE) is entirely deprived of such interaction inside the transpososome. Our work suggests that formation of the Hermes synaptic complex is sequential and relies on the considerable difference of affinity of the transposase towards its transposon ends. Thus, we propose that Hermes dimers multimerize to gather enough BED domains to find the LE among the abundant genomic DNA, facilitating the subsequent interaction with the RE, most likely solely based on recognition of its terminal inverted repeat.