Evidence for a DNA-relay mechanism in ParABS-mediated chromosome segregation

Abstract

The widely conserved ParABS system plays a major role in bacterial chromosome segregation. How the components of this system work together to generate translocation force and directional motion remains uncertain. Here, we combine biochemical approaches, quantitative imaging and mathematical modeling to examine the mechanism by which ParA drives the translocation of the ParB/parS partition complex in Caulobacter crescentus. Our experiments, together with simulations grounded on experimentally-determined biochemical and cellular parameters, suggest a novel 'DNA-relay' mechanism in which the chromosome plays a mechanical function. In this model, DNA-bound ParA-ATP dimers serve as transient tethers that harness the elastic dynamics of the chromosome to relay the partition complex from one DNA region to another across a ParA-ATP dimer gradient. Since ParA-like proteins are implicated in the partitioning of various cytoplasmic cargos, the conservation of their DNA-binding activity suggests that the DNA-relay mechanism may be a general form of intracellular transport in bacteria.DOI: http://dx.doi.org/10.7554/eLife.02758.001

Directed and persistent movement arises from mechanochemistry of the ParA/ParB system

“…2-5). The transient tethering aspect of the model provides an explanation for the apparent reduced diffusion constants measured in both in vitro and in vivo experiments (14,24,35).…”

“…Recently, a "DNA relay" model proposed that, for the Caulobacter crescentus ParABS system, DNA-bound ParA-ATP dimers tether cargo to the bacterial chromosome but require chromosomal elastic dynamics to generate the translocation force that "relays" cargo from one DNA region to another (35). This mechanical contribution from the chromosome was evoked because simulations of a simple "diffusion-binding" model indicated that the cargo diffused too slowly to produce directed cargo movement (35).…”

Surfing on Protein Waves: Proteophoresis as a Mechanism for Bacterial Genome Partitioning

“…In the case of a surface interaction, for which the volume is limited to the boundaries of the surface complex, α c would thus take much higher values. This argument can be easily generalized to higher dimensions D. Our approach also allows us to clarify analytically the physical mechanism at play, by going beyond the numerical simulations usually performed in a limited range of parameters, and to show explicitly that other effects like polymerization [12] and DNA elasticity [13,14] are not needed to account for segregation.…”

“…This process entails the oscillation of ParA from pole to pole and the separation of the ParBS partition complex into two complexes with distinct sub-cellular trajectories and long-term localization. Overall, these interactions result in an equidistant, stable positioning of the duplicated DNA molecules along the cell axis.The specific modeling of ParABS systems falls into two categories: either "filament" (pushing/pulling the cargos, similar to eukaryotic spindle apparatus [3]) or reaction-diffusion models [8][9][10][11][12][13][14][15]. Recent superresolution microscopy experiments have been unable to observe filamentous structures of ParA [5,13], disfavoring polymerization-based models [12].…”

“…Recent superresolution microscopy experiments have been unable to observe filamentous structures of ParA [5,13], disfavoring polymerization-based models [12]. Reaction-diffusion models have been mainly investigated numerically to describe experimental observations like single or multiple ParBS complex positioning.…”