Para Protein Pattern Formation Drives Bacterial Plasmid Segregation

Abstract

DNA segregation is an essential process that ensures that every daughter cell inherits a copy of genomic DNA. Many bacterial chromosomes and low-copy-number plasmids such as P1 in Escherichia coli have partitioning systems to separate and transport genomes towards opposite cell-halves before cell division. Only three components are required to partition plasmids: a parS sequence on the plasmid and two proteins, ParA and ParB. It was widely believed that P1 plasmids segregate similar to a mitotic mechanism in eukaryotes. Here, we propose that the partition proteins pattern the bacterial nucleoid using it as a track for plasmid motion. We reconstituted the P1 partition system in a DNA-coated flowcell to mimic an artificial nucleoid, and visualized the dynamics with TIRF microscopy. ParA and ParB coated the DNA surface uniformly. ParB binds specifically to parS site on the plasmid and stimulated the disassembly of ParA, an ATP-dependent DNA-binding protein. This generated a transient depletion zone surrounding the plasmid prior to its motion on the DNA surface. FRAP measurements showed the partition proteins exchanging rapidly on DNA without forming any filamentous structures. Our results support a reaction-diffusion based mechanism, where ParB on the plasmid chases and redistributes ParA patterns on the nucleoid and in turn drives plasmid movement.