Assembly of the mariner Mos1 synaptic complex.

The mobility of transposable elements via a cut-and-paste mechanism depends on the elaboration of a nucleoprotein complex known as the synaptic complex. We show here that the Mos1 synaptic complex consists of the two inverted terminal repeats of the element brought together by a transposase tetramer and is designated paired-end complex 2 (PEC2). The assembly of PEC2 requires the formation of a simpler complex, containing one terminal repeat and two transposase molecules and designated single-end complex 2 (SEC2). In light of the formation of SEC2 and PEC2, we demonstrate the presence of two binding sites for the transposase within a single terminal repeat. We have found that the sequence of the Mos1 inverted terminal repeats contains overlapping palindromic and mirror motifs, which could account for the binding of two transposase molecules "side by side" on the same inverted terminal repeat. We provide data indicating that the Mos1 transposase dimer is formed within a single terminal repeat through a cooperative pathway. Finally, the concept of a tetrameric synaptic complex may simply account for the inability of a single mariner transposase molecule to interact at the same time with two kinds of DNA: the inverted repeat and the target DNA.