DNA/polysome phase separation and cell width confinement couple nucleoid segregation to cell growth in Escherichia coli

Chromosome segregation is essential for cellular proliferation. Unlike eukaryotes, bacteria lack cytoskeleton-based machinery to segregate their chromosomal DNA (nucleoid). The bacterial ParABS system segregates the duplicated chromosomal regions near the origin of replication. However, this function does not explain how bacterial cells partition the rest (bulk) of the chromosomal material. Furthermore, some bacteria, including Escherichia coli , lack a ParABS system. Yet, E. coli faithfully segregates nucleoids across various growth rates. Here, we provide theoretical and experimental evidence that polysome production during chromosomal gene expression helps compact, split, segregate, and position nucleoids in E. coli through phase separation, inherently coupling these processes to biomass growth across nutritional conditions. Halting polysome formation immediately stops sister nucleoid migration while ensuing polysome depletion gradually reverses nucleoid segregation. Redirecting gene expression away from the chromosome and toward plasmids arrests nucleoid segregation and causes ectopic polysome accumulations that drive aberrant nucleoid dynamics. Cell width perturbations show that radial confinement of polysomes and nucleoids spatially controls their phase separation to ensure that nucleoids split along the cell width and segregate along the cell length. Our findings suggest a built-in mechanism for coupling chromosome segregation to cell growth and highlight the importance of cell width regulation in nucleoid segregation.