Scientists Terence Strick, Tatsuhiko Kawaguchi and Tatsuya Hirano of Cold Spring Harbor Laboratory employed a nanomanipulation technique by which small individual molecules of DNA, tethered on one end to a glass slide and attached on the other end to a magnetic bead, could be gently stretched and twisted using small magnets. The technique allowed the researchers to exert controlled, variable force on the extended DNA, directly measuring changes in its compaction following interactions with Condensin complexes isolated from frog eggs. Because the helical DNA could be twisted, the scientists were also able to investigate how DNA topology – in this case, topological states called positive and negative supercoiling – might affect its ability to be compacted by Condensin. Such measurements are central to illuminating the molecular mechanism used by Condensin in the cell.
The researchers found that Condensin compacts DNA against a weak stretching force, but that increasing the force on the DNA reversed compaction, effectively breaking apart the molecular interactions formed by Condensin. Carefully measuring changes in distance between the two ends of the DNA molecule revealed evidence that both compaction and decompaction often occurred in jumps of certain lengths. Comparing the range of these step sizes to the physical dimensions of Condensin complexes, the authors were able to make some informed proposals for how Condensins interact with DNA – for example, by forming large DNA loops that can be popped open by increased stretching force. It remains unclear whether individual Condensin complexes can accomplish this task single-handedly, or whether multiple complexes act cooperatively, but the new findings and techniques employed here establish a solid foundation for further work on such questions.
Terence R. Strick, Tatsuhiko Kawaguchi and Tatsuya Hirano: "Real-time Detection of Single-molecule DNA Compaction by Condensin I"
Published online in Current Biology 22 April 2004. Appearing in print in Current Biology Volume 14, Number 10, 25 May 2004.
Journal
Current Biology