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The discovery of the prokaryotic cytoskeleton has revolutionized our thinking about spatial organisation in prokaryotes. However, the roles different bacterial cytoskeletal proteins play in the localisations of diverse biomolecules are controversial. Bacterial chemotaxis depends on signalling through large protein clusters and each cell must inherit a cluster on cytokinesis. In Escherichia coli the membrane chemosensory clusters are polar and new static clusters form at pre-cytokinetic sites, ensuring positioning at new poles after cytokinesis and suggesting a role for the bacterial FtsZ and MreB cytoskeletons. Rhodobacter sphaeroides has both polar, membrane-associated and cytoplasmic, chromosome-associated chemosensory clusters. This study sought to investigate the roles of FtsZ and MreB in the partitioning of the two chemosensory clusters in R. sphaeroides. The relative positioning between the two chemosensory systems, FtsZ and MreB in R. sphaeroides cells during the cell cycle was monitored using fluorescence microscopy. FtsZ forms polar spots after cytokinesis, which redistribute to the midcell forming nodes from which gradients of FtsZ extend circumferentially to form the Z-ring. The proposed node-precursor model might represent a common mechanism for the formation of cytokinetic rings. The MreB cytoskeleton continuously reorganizes between patchy and filamentous structures, and colocalises with FtsZ at midcell. Membrane chemosensory proteins form individual dynamic unit-clusters with mature clusters containing about 1000 CheW3 proteins. These unit-clusters diffuse randomly within the membrane but have a higher propensity for curved regions like cell poles. Membrane clusters do not colocalise with FtsZ and MreB and appear excluded from the Z-ring vicinity. The bipolar localisation of membrane clusters is established after cell division via random diffusion and polar trapping of clusters. The cytoplasmic chemosensory clusters colocalise with FtsZ at midcell in new-born cells. Before cytokinesis one cluster moves to a daughter cell, followed by the second moving to the other cell. FtsZ and MreB do not participate in the positioning of cytoplasmic clusters. Therefore the two homologous chemosensory clusters use different mechanisms to ensure partitioning, and neither system utilizes FtsZ or MreB for positioning. |
