COMMENTARY COMMENTARY Plasmid segregation by a moving ATPase gradient Daniela Kiekebuscha,b,c and Martin Thanbichlera,b,c,1 aMax Planck Research Group “Prokaryotic Cell Biology”, Max Planck Institute for Terrestrial Microbiology, 35043 Marburg, Germany; bFaculty of Biology, Philipps-Universität, 35043 Marburg, Germany; and cLOEWE Center for Synthetic Microbiology, 35043 Marburg, Germany Unlike the mitotic segregation of eukaryotic called “partition complex” (8). This complex sister chromatids, DNA partitioning in bac- then dynamically interacts with ParA to drive teria is still not well understood. Bacterial the directed movement of the DNA cargo high–copy-number plasmids can be stably (Fig. 1). In the presence of ATP, ParA asso- maintained by random distribution of their ciates nonspecifically with DNA and exhibits copies during cell division. In contrast, the a weak intrinsic ATPase activity that is stim- faithful transmission of low–copy-number ulated synergistically by ParB and DNA (9– plasmids and many chromosomes depends 11). Moreover, several biochemical studies on an active process mediated by conserved, have shown that ParA can assemble into fil- tripartite segregation systems (1). A central amentous structures upon ATP binding (6, component of these machineries is a nucleo- 10, 12, 13). This finding, together with anal- side triphosphatase driving the partition- yses of ParA localization in vivo, has pro- ing reaction, which can be classified as an vided the basis for the filament-pulling actin-like ATPase (ParM), a tubulin-like model of DNA segregation (7). However, GTPase (TubZ), or a Walker-type ATPase the physiological relevance of ParA polymer- (ParA). Systems using an actin or tubulin ization is highly controversial (14, 15). In homolog function by means of a filament- particular, recent studies investigating the E. based pushing or pulling mechanism (2, coli P1 and F plasmid segregation systems – 3). Most low copy-number plasmids and have cast serious doubt on a role of filament Fig. 1. (A) Partitioning (sop)locusoftheE. coli F chromosomes are, however, segregated formation in the partitioning process (5, plasmid, comprising genes for an ATPase (SopA) and by Walker-type ATPases. Despite exten- 16–18). Instead, ParA was proposed to act an adaptor protein (SopB), as well as a centromere-like sive research, it is not yet unambiguously by a diffusion-ratchet mechanism, which region including multiple tandem repeats of the SopB binding site (sopC). (B)Thesop locusisessentialfor established how this third group of proteins includes the following steps: ParA-ATP the faithful segregation of F plasmids to the future harnesses the energy released during ATP dimers bind nonspecifically to chromo- daughter-cell compartments. Wild-type plasmids are reg- hydrolysis for plasmid movement. Based somal DNA and transiently tether plas- ularly positioned over the nucleoid (Upper), whereas on previous analyses, two competing mids to the nucleoid surface through plasmids lacking the sop locus are concentrated in –parS the polar regions of the cell (Lower). (C)Mechanism models have been put forward. In the fil- interaction with the ParB partition com- of sop-mediated plasmid movement. Nucleoid-asso- ament-pulling model, ParA is assumed to plex (17). In the resulting quaternary com- ciated SopA-ATP dimers bind to a SopB–sopC parti- form polymers that move DNA by repeated plex, ParB stimulates the ATPase activity of tion complex, thereby immobilizing a copy of the F polymerization/depolymerization cycles. ParA, thereby inducing its release from the plasmid on the nucleoid surface. After stimulation of In contrast, the diffusion-ratchet model DNA. Because reactivation of ParA involves its ATPase activity by SopB, SopA dissociates from the DNA, leaving behind a zone of low SopA con- proposes a concentration gradient of ParA a series of slow conformational changes, it is centration (Upper). As a consequence, the plasmid dimers on the nucleoid as the driving force unable to immediately reassociate with the starts to diffuse and then reattaches to the nucleoid for DNA segregation. In PNAS, Vecchiarelli nucleoid (17). This lag creates a ParA de- through association with neighboring SopA molecules Lower et al. (4) now provide direct evidence in pletion zone in the vicinity of the partition ( ). Iteration of these steps results in the directed movement of plasmid copies across the nucleoid support of the latter model by fully recon- complex, which ultimately results in the de- surface. stituting in vitro the segregation system of the tachment of the plasmid from the nucleoid Escherichia coli Fplasmid. surface. After its dissociation, the plasmid dif- Plasmid segregation systems based on fuses in a stochastically chosen direction. As central point of the model, the initial direc- Walker-type ATPases position plasmid cop- it reaches the edge of the depletion zone, it tion taken by the plasmid is reinforced by ies at regular distances over the nucleoid encounters an increasing number of nucleoid- (5–7). In addition to the ATPase compo- bound ParA dimers, which make new con- Author contributions: D.K. and M.T. wrote the paper. nent (ParA), they comprise a centromere-like tacts to the plasmid partition complex. Mov- The authors declare no conflict of interest. parS DNA sequence ( ) and an adaptor protein ing along this ParA gradient, the plasmid is See companion article 10.1073/pnas.1401025111. parS (ParB). Binding of ParB to motifs on the finally immobilized again and initiates the 1To whom correspondence should be addressed. E-mail: DNA cargo results in the formation of a so- formation of a new depletion zone. As a [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1402867111 PNAS Early Edition | 1of2 Downloaded by guest on September 28, 2021 low ParA concentrations in the wake of the narrow space between the nucleoid surface In doing so, they tracked along a moving partition complex, thereby giving rise to ro- and the cytoplasmic membrane, a situation SopA gradient, leaving behind a region of bust, unidirectional movement of plasmid that greatly facilitates repeated interactions low SopA concentration that was slowly molecules (Fig. 1C). of ParB–parS partition complexes with nucle- refilled with SopA molecules from solution. As a first step toward verifying the diffu- oid-bound ParA molecules. Therefore, in line It still needs to be determined if, analogous to sion-ratchet model, the P1 and F plasmid with the experimental results, spatial con- P1 ParA, a time-delay between ATP binding segregation systems have recently been re- finement was proposed to be another key and reassociation with DNA is involved in constituted in a cell-free set-up (16, 18). To requirement for ParA-mediated plasmid the emergence of the SopA gradient. Interest- mimic the nucleoid surface, the bottom of ingly, not all beads exhibited directional a 25-μm-thick microfluidic flow cell was The study by Vecchiarelli movement. Whereas directed beads consis- densely coated with nonspecific DNA frag- et al. significantly tently stayed in close contact with the sur- ments. The flow cell was then loaded with a face, others diffused freely along the surface mixture of ParA-GFP, ParB, and a fluores- advances our under- and “bounced” in and out of the TIRFM cently labeled parS-bearing plasmid prepared standing of bacterial illumination area, ruling out a significant in an ATP-containing buffer. Subsequently, DNA segregation. contribution of the magnetic force to the the dynamics of the components were visu- straight motion of directed beads. alized by total internal reflection microscopy segregation by a diffusion-ratchet mechanism Although the characteristics of a magnetic (TIRFM), a technique that specifically re- (16, 18). bead differ from those of a DNA molecule, solves surface-associated processes. The two In the present study, Vecchiarelli et al. (4) the study by Vecchiarelli et al. (4) signifi- systems exhibited similar dynamics, suggest- have verified this hypothesis by reinvesti- cantly advances our understanding of bacte- ing a comparable segregation mechanism. gating the dynamics of F plasmid segregation rial DNA segregation. The work identifies the Initially, plasmids were stably tethered to using an advanced version of their cell-free partitioning reaction driven by (at least a sub- a layer of ParA-GFP associated with the system. The plasmids used in the previous set of) Walker-type ATPases as a diffusion- DNA carpet. After some time, they started to studies were replaced by magnetic beads regulated process, and thus adds to the growing move around their anchor point in a circular coated with fluorescently labeled sopC- body of evidence suggesting that the physio- Brownian motion, leading to the displacement containing DNA. Spatial confinement was logical relevance of filament formation by of ParA-GFP in their surroundings. Eventu- then simulated by application of a magnetic these proteins may have been overestimated. ally, the plasmids dissociated from the sur- force perpendicular to the DNA-coated flow It will be interesting to perform the same face and diffused away, leaving behind cell surface. This experimental set-up not kind of analysis on other ParA-dependent a zone depleted of ParA-GFP molecules. only reproduced the previously observed P1 segregation systems that have been proposed These observations strikingly mirrored the and F plasmid dynamics, but also made it to use a polymerization-based mechanism in vivo dynamics of the P1 plasmid (5)