Putting the Par back into ParABS: Plasmid Partitioning Driven by ParA Oscillations

Abstract

AbstractThe ParABS system is crucial for the faithful segregation and inheritance of many bacterial chromosomes and low-copy number plasmids. However, despite extensive research, the spatio-temporal dynamics of the ATPase ParA and its connection to the dynamics and positioning of the ParB-coated cargo has remained unclear. In this study, we utilise high-throughput imaging, quantitative data analysis, and computational modelling to explore thein vivodynamics of F-plasmid ParA and its interaction with ParB-coated plasmids and the nucleoid. As previously observed, we find that ParA undergoes collective migrations (‘flips’) between cell halves multiple times per cell cycle, resulting in oscillations over time. We reveal that a constricted nucleoid is required for these migrations and that they are triggered by a plasmid crossing into the cell half with greater ParA. Using simulations, we show that these dynamics can be explained by the combination of nucleoid constriction and cooperative ParA binding to the DNA, in line with the behaviour of other ParA proteins. We further show that these ParA flips act to equally partition plasmids between the two lobes of the constricted nucleoid and are therefore important for plasmid stability, especially in fast growth conditions for which the nucleoid constricts early in the cell cycle. Overall our work sheds light on a second mode of action of the ParABS system and deepens our understanding of this important system.