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

ABSTRACT: The segregation of DNA before cell division is essential for faithful genetic inheritance. In many bacteria, segregation of low-copy number plasmids involves an active partition system composed of a nonspecific DNA-binding ATPase, ParA, and its stimulator protein ParB. The ParA/ParB system drives directed and persistent movement of DNA cargo both in vivo and in vitro. Filament-based models akin to actin/microtubule-driven motility were proposed for plasmid segregation mediated by ParA. Recent experiments challenge this view and suggest that ParA/ParB system motility is driven by a diffusion ratchet mechanism in which ParB-coated plasmid both creates and follows a ParA gradient on the nucleoid surface. However, the detailed mechanism of ParA/ParB-mediated directed and persistent movement remains unknown. Here, we develop a theoretical model describing ParA/ParB-mediated motility. We show that the ParA/ParB system can work as a Brownian ratchet, which effectively couples the ATPase-dependent cycling of ParA-nucleoid affinity to the motion of the ParB-bound cargo. Paradoxically, this resulting processive motion relies on quenching diffusive plasmid motion through a large number of transient ParA/ParB-mediated tethers to the nucleoid surface. Our work thus sheds light on an emergent phenomenon in which nonmotor proteins work collectively via mechanochemical coupling to propel cargos-an ingenious solution shaped by evolution to cope with the lack of processive motor proteins in bacteria.

SUBMITTER: Hu L

PROVIDER: S-EPMC4697391 | biostudies-other | 2015 Dec

REPOSITORIES: biostudies-other

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Project description:Representatives of two families of bacterial Par proteins, ParA and ParB, are encoded by the majority of bacterial chromosomes in the close vicinity of oriC. ParA(Soj) and ParB(Spo0J) proteins of Pseudomonas aeruginosa are both important for optimal growth, nucleoids segregation, cell division and different types of motility. Comparative transcriptome analysis of parAnull, parBnull mutants versus parental PAO1161 strain of P. aeruginosa demonstrated global changes in genes expression pattern in logarithmic phase of planktonic cultures grown on rich medium. The set of genes that were similarly regulated in both mutant strains as compared to the wild-type strain as well as two sets of genes uniquely affected in the particular mutant were defined suggesting that ParA and ParB may act in common and independently. In general, many genes involved in cell division, DNA and RNA processing and metabolic processes were down-regulated in mutant cells, in contrast genes which products play a role in adaptation, protection, motility, cell-to-cell signaling as well as signal transduction increased their expression in par mutant cells. Besides their role in chromosome segregation, ParA and ParB seem to have the potential to regulate genes transcription. The altered expression of a large number of genes encoding known or predicted transcriptional regulators and genes coding for products involved in c-di-GMP signalling, suggests that the part of observed global changes in genes expression pattern in parAnull and parBnull mutants might be the effect of indirect regulation mediated by regulatory genes under ParA and ParB control. The extended regulatory network provides the mechanism to modulate genes expression in response to the stage of the chromosome segregation process and cell cycle. Pseudomonas aeruginosa PAO1161 (leu, r-, RifR), derivative of PAO1, as a control (reference) strain, Pseudomonas aeruginosa PAO1161 parA1-40::smh (parAnull) and Pseudomonas aeruginosa PAO1161 parB1-18::TcR (parBnull) disruption mutant strains were used in the experiments. Three independent biological replicates of total RNA were isolated for each strain from logarithmic (Log) phase of planktonic culture grown on rich medium (L broth) at 37oC. In total, nine samples of RNA were prepared.

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

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