Aspects of chromosome compaction have been well characterized, yet little is known about the higher orders of DNA compaction. However, two papers — from the Hirano group in Current Biology and from Bustamante and colleagues in Science — now describe the compaction of DNA by the Xenopus laevis condensin-I complex and by the Escherichia coli condensin MukBEF, respectively.
Using single-molecule manipulation techniques, the Hirano group showed that DNA is reversibly compacted against a weak stretching force in the presence of condensin I and hydrolysable ATP. Increasing the stretching force resulted in large, discrete increases in the DNA end-to-end extension, and the size of the increases led this group to propose that condensin I compacts DNA by introducing loops along the DNA. Also using single-molecule techniques, Bustamante and co-workers showed that MukBEF cooperatively compacts DNA into a repetitive, stable structure in an ATP-binding-dependent manner. Extending the DNA caused it to expand in a series of repetitive steps, which, surprisingly, were identical in every subsequent experiment. In addition, DNA compaction after this extension occurred in the absence of ATP and free MukBEF. Consequently, these authors propose that MukBEF requires ATP binding to form force-insensitive, intermolecular contacts and to polymerize along DNA. In addition, they propose that force-sensitive, intramolecular contacts between the two heads of each MukBEF molecule loop the DNA and induce compaction. REFERENCES Strick, T. R. et al. Real-time detection of single-molecule DNA compaction by condensin I. Curr. Biol. 14, 874–880 (2004) Case, R. B. et al. The bacterial condensin MukBEF compacts DNA into a repetitive, stable structure. Science 3 June 2004 (doi:10.1126/science.1098225)
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