Biochemistry

Plasmid Propulsion

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Science  05 Sep 2008:
Vol. 321, Issue 5894, pp. 1273
DOI: 10.1126/science.321.5894.1273a

To be propagated stably in prokaryotes, low-copy number plasmids must be allocated actively during cell division. The R1 plasmid is maintained at four to six copies per cell by the par operon, which encodes the DNA-binding protein ParR and the actin-like ATPase ParM. ParR binds cooperatively as a dimer to 11-base pair repeats in parC; ParM undergoes ATP-dependent polymerization, but only grows into long parallel filaments that are capable of pushing replicated plasmids apart when capped by the ParR-parC complex. To understand how elongating filaments are stabilized, Salje and Lowe have used electron microscopy and biochemistry to determine the architecture of capped filaments. ParR-parC complexes have previously been shown to form a clamplike structure in which parC DNA wraps around a helical array of ParR dimers. Guided by biochemical mapping of the ParR-ParM interaction sites, they modeled the crystal structure of ParR onto the end of the double-helical ParM filament. The ParR-parC clamp wraps around the filament with the C-terminal regions of ParR bound to exposed loops of ParM. Each ParR-parC complex binds the end of a single filament, and the filament ends can be bound simultaneously. Unlike actin, ParM forms left-handed filaments, which allows ParM monomers access to the ends of protofilaments capped with right-handed ParR-parC. The authors suggest a model in which force is produced by the alternating addition of monomers to each protofilament accompanied by rocking of the ParR clamp from side to side, analogous to the model proposed for formin-assisted actin polymerization. — VV

EMBO J. 27, 2230 (2008).

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