A moving ParA gradient on the nucleoid directs subcellular cargo transport via a chemophoresis force.

DNA segregation is a critical process for all life, and although there is a relatively good understanding of eukaryotic mitosis, the mechanism in bacteria remains unclear. The small size of a bacterial cell and the number of factors involved in its subcellular organization make it difficult to study individual systems under controlled conditions in vivo. We developed a cell-free technique to reconstitute and visualize bacterial ParA-mediated segregation systems. Our studies provide direct evidence for a mode of transport that does not use a classical cytoskeletal filament or motor protein. Instead, we demonstrate that ParA-type DNA segregation systems can establish a propagating ParA ATPase gradient on the nucleoid surface, which generates the force required for the directed movement of spatially confined cargoes, such as plasmids or large organelles, and distributes multiple cargos equidistant to each other inside cells. Here we present the critical principles of our diffusion-ratchet model of ParA-mediated transport and expand on the mathematically derived chemophoresis force using experimentally-determined biochemical and cellular parameters.