Biochemistry

Fit to Function

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Science  12 Jun 2009:
Vol. 324, Issue 5933, pp. 1366
DOI: 10.1126/science.324_1366b
CREDIT: DUNHAM ET AL., EMBO J. 28, 10.1038/EMBOJ.2009.120 (2009)

In bacteria, low–copy number plasmids provide a model system for studying the segregation of DNA into daughter cells—partitioning requires only a centromere-like DNA site, a partition NTPase, and a centromere-binding protein. The type II systems use the actinlike partition protein ParM to drive plasmid separation. The Escherichia coli plasmid P1 uses the type I system of the partition ATPase ParA, the centromere binding protein ParB, and the DNA site parS. ParA with ATP bound interacts with centromere-bound ParB to mediate segregation; however, ADP-bound ParA acts as a transcription factor that represses parAB transcription.

Using crystallography and biochemistry, Dunham et al. found that ParA forms a conformationally flexible dimer that relies on an interaction involving the N-terminal α helix. ADP binding locked the monomers (blue, yellow) into agreeable conformations for DNA binding and induced the folding of two positively charged regions (red) that docked onto the DNA (gray). In contrast, electron microscopy showed that ATP-bound ParA forms filaments that likely facilitate segregation.

EMBO J. 28, 10.1038/emboj.2009.120 (2009).

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