Right-Handed Nucleosome: Myth or Reality?

In their recent paper in Cell, Furuyama and Henikoff (2009) report that nucleosomes in centromeres may be right-handed, that is, they wrap DNA in a right-handed manner and induce positive supercoils. This raises intriguing new questions, such as how centromeric histone variants may be assembled into right-handed particles, and why chromatin would retain negative supercoiling in chromosome arms but positive supercoiling in centromeres. We wish to comment on these new findings in the context of topological insights that we have gained from recent in vitro experiments with centromeric nucleosomes and single chromatin fibers submitted to torsional constraints and from 3D modeling of chromatin dynamics.