The Movement of Tn3‐Like Elements: Transposition and Cointegrate Resolution

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Abstract
This chapter discusses the organization of Tn3-family transposons and their transposition (the formation and resolution of the cointegrate intermediate). The bulk of the chapter, however, is concerned with the process of cointegrate resolution as performed by the resolvase (serine recombinase) class of site-specific recombinases, since this is both the most novel feature of Tn3-family transposons and the feature about which we know the most. While initial studies failed to identify the resolvase, they led to the proposal that cointegrates were an intermediate in the complete transposition pathway and that an internal site, deleted in this subset, was needed for the conversion of cointegrates to simple insertions. This proposal was subsequently proved correct, and it became apparent that Tn3 intermolecular transposition proceeded in two distinct and separable steps: formation of a cointegrate, involving the combined action of the transposase and the host cell replication machinery, followed by cointegrate resolution using the distinct site-specific recombination activity of resolvase. Taking a slightly different perspective (from that of the resolution systems), two transposons with very similar resolvases, Tn5501 and Tn(pTF5), have dramatically different transposases, suggesting that two different transposons capture the same resolvase system. Finally, despite their similar organization, two TnpI-encoding transposons, Tn4430 and Tn5401, have transposases and TnpI recombinases that are so diverged that independent origins seem highly likely.