  Using magnetic tweezers to investigate the mechanical response of single
chromatin fibers, we show that fibers submitted to large positive torsion
transiently trap positive turns, at a rate of one turn per nucleosome. A
comparison with the response of fibers of tetrasomes (the (H3-H4)2 tetramer
bound with ~50 bp of DNA) obtained by depletion of H2A-H2B dimers, suggests
that the trapping reflects a nucleosome chiral transition to a metastable form
built on the previously documented righthanded tetrasome. In view of its low
energy, <8 kT, we propose this transition is physiologically relevant and
serves to break the docking of the dimers on the tetramer which in the absence
of other factors exerts a strong block against elongation of transcription by
the main RNA polymerase.
