Bacterial mitosis: partitioning protein ParA oscillates in spiral‐shaped structures and positions plasmids at mid‐cell

Abstract

The par2 locus of Escherichia coli plasmid pB171 encodes oscillating ATPase ParA, DNA binding protein ParB and two cis‐acting DNA regions to which ParB binds (parC1 and parC2). Three independent techniques were used to investigate the subcellular localization of plasmids carrying par2. In cells with a single plasmid focus, the focus located preferentially at mid‐cell. In cells with two foci, these located at quarter‐cell positions. In the absence of ParB and parC1/parC2, ParA‐GFP formed stationary helices extending from one end of the nucleoid to the other. In the presence of ParB and parC1/parC2, ParA‐GFP oscillated in spiral‐shaped structures. Amino acid substitutions in ParA simultaneously abolished ParA spiral formation, oscillation and either plasmid localization or plasmid separation at mid‐cell. Therefore, our results suggest that ParA spirals position plasmids at the middle of the bacterial nucleoid and subsequently separate them into daughter cells.