A model for chromosome organization during the cell cycle in live E. coli

Area of Science:

• Microbiology
• Molecular Biology
• Biophysics

Background:

• Bacterial chromosomal DNA forms a compact nucleoid, but its organization in live bacteria remains poorly understood.
• Existing methods have limitations in monitoring the complex DNA organization within live bacterial cells.

Purpose of the Study:

• To investigate the auto-packaging behavior of circular DNA under mechanical stress.
• To propose and test a dynamic model for bacterial chromosomal DNA organization during the cell division cycle.

Main Methods:

• Atomic force microscopy was used to analyze the conformation of circular plasmid DNA under mechanical stress.
• Basic physical principles were applied to deduce the mechanism of DNA auto-packaging.
• Live E. coli cell cycles were monitored to test a proposed dynamic model of DNA organization.

Main Results:

• Circular plasmid DNA was observed to auto-package into a uniform dual-toroidal-spool conformation when subjected to mechanical stress (bending and un-winding).
• A dynamic model for E. coli chromosomal DNA organization during cell division was proposed based on these observations.
• Experimental monitoring of HNS clusters in live E. coli showed results consistent with the proposed dynamic model.

Conclusions:

• The study elucidates a mechanism for circular DNA auto-packaging driven by mechanical stress.
• A novel dynamic model for bacterial nucleoid organization during the cell cycle is proposed and validated.
• The model successfully explains many previously observed features of bacterial nucleoids in vivo.