Multiple sequence elements bound by specific proteins are a common feature of many sites of action on DNA, such as promoters, replication origins, and recombination loci, but the mechanisms of protein assembly on these DNA sites are not well understood. Mu transposition is an ideal system to study these mechanisms because the transposition complexes are very stable and multiple DNA sites are specifically required for their assembly. How binding of the transposase protein to the DNA sites triggers assembly of the transposase tetramer is being investigated. Our current model for this process is that DNA binding triggers a conformational change in transposase to uncover regions of the protein involved in the subunit interfaces. Transposition reactions can be divided into two general classes, nonreplicative and replicative, depending on how much DNA synthesis accompanies a round of recombination.
Most elements use exclusively one pathway or the other.
Mu, however, moves via a nonreplicative mechanism when it first infects a cell but uses a replicative mechanism during lytic growth. The replicative pathway has been analyzed in depth, but little is known about the mechanism of nonreplicative transposition or what controls the switch between these two pathways. We have evidence supporting the hypothesis that nonreplicative transposition proceeds through a set of intermediates that is distinct from the replicative pathway. These results provide the foundation for analyzing the mechanism of nonreplicative transposition and how the pathway switch is controlled. Control of insertion site choice is an especially interesting aspect of DNA transposition.