Fig. 1. Proposed IS911 transposition pathway. Transposon DNA (bold lines), donor backbone sequences (fine lines), target DNA (dotted lines), IRL and IRR (small circles). The different steps of IS911 transposition are shown: synapsis of the ends in a plasmid donor to generate synaptic complex A; strand cleavage at one end (the donor end) and transfer to the other (the target end) to form a figure-of-eight structure in a reaction that requires only the transposase OrfAB; second strand resolution requiring host functions but no IS911 proteins to generate the transposon circle; synapsis with a target DNA molecule (synaptic complex B); cleavage of the IRR–IRL junction; and transfer into the target, which requires the transposase OrfAB. Final insertion requires OrfAB and is greatly stimulated by OrfA.