Multicopy plasmids are clustered and localized in Escherichia coli

Area of Science:

• Microbiology
• Molecular Biology
• Cell Biology

Background:

• Multicopy plasmids are essential tools in molecular biology research.
• Previous understanding suggested random diffusion of plasmids within bacterial cells.

Purpose of the Study:

• To investigate the intracellular localization and partitioning dynamics of multicopy plasmids in Escherichia coli.
• To challenge the prevailing model of random plasmid diffusion.

Main Methods:

• Localization of multicopy plasmid RK2 using fluorescent tagging in Escherichia coli.
• Time-lapse microscopy to observe plasmid partitioning.
• Localization of a high-copy-number plasmid derivative (pUC19) using GFP-LacI system.

Main Results:

• The number of observed plasmid foci was significantly lower than the plasmid copy number, indicating clustering.
• Plasmids formed one to two clusters, often localized at midcell or quarter positions.
• Plasmid cluster number correlated with cell length and growth rate, and decreased in stationary phase.
• Plasmid partitioning occurred through the splitting and rapid separation of foci.

Conclusions:

• Multicopy plasmids do not diffuse randomly but exist in clustered structures within bacterial cells.
• Plasmid replication and segregation appear coordinated with the bacterial cell cycle.
• A new model is proposed where plasmids are replicated and partitioned in targeted clusters.