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Published online November 26, 2007
doi:10.1083/jcb.200708206
The Journal of Cell Biology, Vol. 179, No. 5, 1059-1066
The Rockefeller University Press, 0021-9525 $30.00
© 2007 Campbell et al.
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Article

 In vivo visualization of type II plasmid segregation: bacterial actin filaments pushing plasmids

Christopher S. Campbell and R. Dyche Mullins

School of Medicine, University of California, San Francisco, San Francisco, CA 94158

Correspondence to R. Dyche Mullins: dyche{at}mullinslab.ucsf.edu

Type II par operons harness polymerization of the dynamically unstable actin-like protein ParM to segregate low-copy plasmids in rod-shaped bacteria. In this study, we use time-lapse fluorescence microscopy to follow plasmid dynamics and ParM assembly in Escherichia coli. Plasmids lacking a par operon undergo confined diffusion with a diffusion constant of 5 x 10–5 µm2/s and a confinement radius of 0.28 µm. Single par-containing plasmids also move diffusively but with a larger diffusion constant (4 x 10–4 µm2/s) and confinement radius (0.42 µm). ParM filaments are dynamically unstable in vivo and form spindles that link pairs of par-containing plasmids and drive them rapidly (3.1 µm/min) toward opposite poles of the cell. After reaching the poles, ParM filaments rapidly and completely depolymerize. After ParM disassembly, segregated plasmids resume diffusive motion, often encountering each other many times and undergoing multiple rounds of ParM-dependent segregation in a single cell cycle. We propose that in addition to driving segregation, the par operon enables plasmids to search space and find sister plasmids more effectively.

Abbreviation used in this paper: MSD, mean squared displacement.


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