Solvent quality and chromosome folding in Escherichia coli

Summary

All cells must fold their genomes, including bacterial cells where the chromosome is compacted into a domain-organized meshwork called nucleoid. Polymer conformation depends highly on the quality of the solvent. Yet, the solvent quality for the DNA polymer inside cells remains unexplored. Here, we developed a method to assess this fundamental physicochemical property in live bacteria. By determining the DNA concentration and apparent average mesh size of the nucleoid, we provide evidence that the cytoplasm is a poor solvent for the chromosome in Escherichia coli. Monte Carlo simulations showed that such a poor solvent compacts the chromosome and promotes spontaneous formation of chromosomal domains connected by lower-density DNA regions. Cryo-electron tomography and fluorescence microscopy revealed that the (poly)ribosome density within the nucleoid is spatially heterogenous and correlates negatively with DNA density. These findings have broad implications to our understanding of chromosome folding and intracellular organization.