Assembly of the active form of the transposase-Mu DNA complex: a critical control point in Mu transposition.

Discovery and characterization of a new intermediate in Mu DNA transposition allowed assembly of the transposition machinery to be separated from the chemical steps of recombination. This stable intermediate, which accumulates in the presence of Ca2+, consists of the two ends of the Mu DNA synapsed by a tetramer of the Mu transposase. Within this stable synaptic complex (SSC), the recombination sites are engaged but not yet cleaved. Thus, the SSC is structurally related to both the cleaved donor and strand transfer complexes, but precedes them on the transposition pathway. Once the active protein-DNA complex is constructed, it is conserved throughout transposition. The participation of internal sequence elements and accessory factors exclusively during SSC assembly allows recombination to be controlled prior to the irreversible chemical steps.