FIGURE 9.

The presynaptic and postsynaptic effect of DinD protein on the process of RecA-mediated homologous DNA strand exchange. RecA-ssDNA nucleoprotein filaments interact with a homologous duplex DNA molecule, pairing the complementary strand (gray) and displacing the identical strand (black). The RecA protein adopts a P conformational state (rectangles) that is distinct from the state when bound only to ssDNA or free RecA monomers (both depicted as ovals). The plectonemic joint molecule intermediate of this reaction contains a region of heteroduplex behind the branch and a region of paranemic association between RecA-ssDNA filaments and the substrate duplex molecule ahead of the branch. Branch migration (to the right) would continue through to the end of the duplex. For assays in vitro, the products are the displaced linear single strand and the heteroduplex molecule (see Fig. 3). DinD protein can affect both the presynaptic and postsynaptic stages of this RecA-mediated process. DinD can prevent synapsis through the sequestration of the duplex DNA substrate. This sequestration effect can also be imposed by a nonspecific duplex DNA-binding protein. Notably, however, DinD can also mediate the disassembly of postsynaptic RecA filaments, whether bound to the heteroduplex product or involved in paranemic associations, perhaps by targeting the P state of RecA filaments.