The Bacterial Nucleoid Drives Cytoplasmic Dynamics

Bacterial cells exhibit complex, cell-cycle-dependent subcellular organization despite the lack of membrane-bound organelles. One of the most popular proposed mechanisms for this cellular ultrastructure is physical exclusion from the dense bacterial nucleoid. To quantitatively investigate this hypothesis, we visualized and mapped the motion of fluorescently-tagged ectopic MS2-mRNA complexes in thousands of growing E. coli cells. We find that the the molecular complexes’ motion strongly depends on their spatial position along the long-axis of the cell and that their dynamics are well characterized by a quantitative model that requires only two physical contributions: nucleoid exclusion and membrane confinement. Strikingly, we also find that the mobility of the molecular complexes is highest in regions of high nucleoid density, and that perturbations to nucleoid structure tend to increase cytoplasmic mobility. These results provide strong quantitative support for two modes of nucleoid action: (1) organizing the cell through physical exclusion forces and (2) as a facilitator of rapid motion throughout the cytoplasm. These results have potentially important biological implications and suggest that the nucleoid may play a much more direct role, than previously thought, in the organization and transport of subcellular components, including large protein complexes and plasmids.