A replication hijacking mechanism for Tn 3 -family replicative transposition
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Transposition of all classes of transposable elements generates DNA intermediates that must be processed by the host to be effective. However, the mechanisms whereby transposons communicate with cellular DNA-processing machineries remain poorly investigated. Here, we provide convergent genetic and biochemical evidence that replicative transposition of the Tn 3 -family transposon Tn 4430 is strictly coupled to replication of the target. Blocking target replication abolishes transposition, while blocking replication of the transposon donor molecule has no effect. Furthermore, the insertion preference of Tn 4430 was found to be altered by the direction of replication and potential replication impediments within the target, suggesting a functional link between the integration mechanism and replication fork progression. In vitro , the transposase TnpA was found to specifically bind to fork-like DNA structures that mimic replication intermediates. Compared to linear DNA fragments, these structures are efficient substrates for TnpA-catalysed end joining. Strand transfer occurred immediately downstream of the fork, poising the transposon for replication. Together, the data suggest a mechanism in which the transposon targets DNA replication intermediates to directly recruit the host replication machinery at the time of transposition. This “replication hijacking” mechanism contrasts with classical “replication hiring” mechanisms during which replication is recruited after strand transfer.
SIGNIFICANCE STATEMENT
Bacterial transposons of the Tn 3 family constitute a threat for human health, being continuously involved in the emergence and spread of new antimicrobial resistances amongst pathogens. The success of these elements is based on their replicative mode of transposition allowing them to produce a new copy of themselves whenever they move. Here, we provide evidence that, rather than recruiting the host replication machinery after integration as is proposed in textbook models for replicative transposition, Tn 3 -family transposons directly transpose into ongoing replication intermediates. We propose that this “replication hijacking” mechanism provides a means of synchronizing transposition with cell replication activity, thus optimizing the movement of transposons with the expansion of bacterial populations.
