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. ▪