Condensin introduces supercoils (left) by wrapping DNA around itself (right).


Packing DNA into a nucleus is no mean feat. Now, David Bazett-Jones (Hospital for Sick Children, Toronto, Canada), Keiji Kimura, and Tatsuya Hirano (Cold Spring Harbor Laboratory, Cold Spring Harbor, NY) have found that a single condensin complex can use ATP to wrap two positive gyres of DNA around itself. That packaging, however, may just be the start.Based on previous experiments, Hirano and colleagues had suggested that an individual condensin might span a considerable distance between DNA binding sites and introduce global writhe that would twist the DNA into a right-handed solenoid. But direct observation of single complexes on naked DNA by electron spectroscopic imaging has now shown that a single complex is instead tightly wrapped with two turns of DNA.

What that means for condensation of cellular chromatin is not yet clear. “We don't think that the local wrapping per se would account for the massive compaction of chromatin,” says Hirano. There are several models that could explain additional compaction. A single condensin could bring two distantly located DNA segments together, although Hirano has no evidence for such a mechanism. A similar outcome could be achieved if multiple condensins bind to each other, or condensin may wind already compacted DNA around its core.

In future experiments, Hirano plans to study the in vitro reaction of condensin with chromatin rather than naked DNA. For now, he favors an old model in which the real function of condensin's wrapping of DNA is the introduction of compensatory negative supercoils in the surrounding DNA. Such superhelical tension might in turn act as a driving force in coiling up a chromatin fiber. ▪


Bazett-Jones, D.P., et al.
. Mol.