Unlike the mitotic segregation of eukaryotic sister chromatids, DNA partitioning in bacteria is still not well understood. Bacterial high–copy-number plasmids can be stably maintained by random distribution of their copies during cell division. In contrast, the faithful transmission of low–copy-number plasmids and many chromosomes depends on an active process mediated by conserved, tripartite segregation systems (1). A central component of these machineries is a nucleoside triphosphatase driving the partitioning reaction, which can be classified as an actin-like ATPase (ParM), a tubulin-like GTPase (TubZ), or a Walker-type ATPase (ParA). Systems using an actin or tubulin homolog function by means of a filamentbased pushing or pulling mechanism (2, 3). Most low–copy-number plasmids and chromosomes are, however, segregated by Walker-type ATPases. Despite extensive research, it is not yet unambiguously established how this third group of proteins harnesses the energy released during ATP hydrolysis for plasmid movement. Based on previous analyses, two competing models have been put forward. In the filament- pulling model, ParA is assumed to form polymers that move DNA by repeated polymerization/depolymerization cycles. In contrast, the diffusion-ratchet model proposes a concentration gradient of ParA dimers on the nucleoid as the driving force for DNA segregation. In PNAS, Vecchiarelli et al. (4) now provide direct evidence in support of the latter model by fully reconstituting in vitro the segregation system of the Escherichia coli F plasmid.
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