Introductory cartoons. (a) The organization of IS607 and its proposed transposition pathway. IS607 and related mobile DNA elements encode a serine transposase, TnpA2, and a second protein of unknown function, TnpB. The transposon ends are cartooned as green triangles. Excision creates a circular intermediate (NDF Grindley, personal communication) and reseals the original host DNA backbone. The circle can then integrate into a new location on the original DNA molecule, or, as cartooned, a new target DNA molecule (blue). (b) Sequences of the IS607 and ISC1904 ends. The deduced overlap dinucleotide at the host DNA/IS element junction (the crossover site) is in bold and underlined (blue); the small repeats that may be specific binding sites for the transposase are in bold (red), and other repeats are underlined (green). The latter repeats may be too far from the crossover dinucleotide for one protomer to interact with both motifs, and could play some other (regulatory?) role. The flanking host DNA sequences (lower case) are different at all insertion sites, and show little obvious pattern. (c) Domain organization of serine recombinases. The conserved catalytic domain of serine recombinases is always found at the N-terminus, except in serine transposases of the IS607 family, which have a MerR-family DNA binding domain at the N-terminus. “Canonical” serine recombinases, which include the resolvases and invertases, have a small C-terminal helix-turn-helix DNA binding domain. “Large” serine recombinases, which include bacteriophage integrases and some transposases, have a much larger C-terminal region that contains two DNA binding domains .