Confinement and crowding control the morphology and dynamics of a model bacterial chromosome

TL;DR

This study models bacterial chromosomes as confined, crowder-influenced polymers, revealing how confinement and crowding affect their shape, size, and dynamics, including helical structures and anomalous diffusion behaviors.

Contribution

It introduces a polymer model capturing chromosome morphology and dynamics under confinement and crowding, aligning with recent experimental observations.

Findings

Chromosome adopts a helical shape within cylindrical confinement.

Crowders compact the chromosome into a nucleoid-like volume.

Chromosome size and shape change non-linearly with cell length.

Abstract

Motivated by recent experiments probing shape, size and dynamics of bacterial chromosomes in growing cells, we consider a polymer model consisting of a circular backbone to which side-loops are attached, confined to a cylindrical cell. Such a model chromosome spontaneously adopts a helical shape, which is further compacted by molecular crowders to occupy a nucleoid-like subvolume of the cell. With increasing cell length, the longitudinal size of the chromosome increases in a non-linear fashion to finally saturate, its morphology gradually opening up while displaying a changing number of helical turns. For shorter cells, the chromosome extension varies non-monotonically with cell size, which we show is associated with a radial to longitudinal spatial reordering of the crowders. Confinement and crowders constrain chain dynamics leading to anomalous diffusion. While the scaling exponent for the mean squared displacement of center of mass grows and saturates with cell length, that of individual loci displays broad distribution with a sharp maximum.