Bacterial chromosome organization by collective dynamics of SMC condensins

TL;DR

This study presents a computational model demonstrating how motor activity of SMC condensins can organize bacterial chromosomes by creating large loops and aligning chromosomal arms, explaining experimental Hi-C contact data.

Contribution

The paper introduces a new physical model showing that motor activity of SMC condensins is crucial for large-scale bacterial chromosome organization.

Findings

Motor slip-links effectively organize entire chromosomes.

Motor activity drives large nested loops in DNA.

Model matches in vivo chromosomal arm alignment observations.

Abstract

A prominent organizational feature of bacterial chromosomes was revealed by Hi-C experiments, indicating anomalously high contacts between the left and right chromosomal arms. These long-range contacts have been attributed to various nucleoid-associated proteins, including the ATPase SMC condensin. Although the molecular structure of these ATPases has been mapped in detail, it still remains unclear by which physical mechanisms they collectively generate long-range chromosomal contacts. Here, we develop a computational model that captures the subtle interplay between molecular-scale activity of slip-links and large-scale chromosome organization. We first consider a scenario in which the ATPase activity of slip-links regulates their DNA-recruitment near the origin of replication, while the slip-link dynamics is assumed to be diffusive. We find that such diffusive slip-links can collectively organize the entire chromosome into a state with aligned arms, but not within physiological constraints. However, slip-links that include motor activity are far more effective at organizing the entire chromosome over all length-scales. The persistence of motor slip-links at physiological densities can generate large, nested loops and drive them into the bulk of the DNA. Finally, our model with motor slip-links can quantitatively account for the rapid arm--arm alignment of chromosomal arms observed in vivo.