Chromatin polymorphism and the nucleosome superfamily: a genealogy.

Nucleosomes were discovered more than thirty years ago as the basic repeating units of chromatin. Since then, nucleosomes have progressively revealed their taste to come in many appearances, upon either adjunction of other proteins (e.g., a fifth histone or a nonhistone protein, HMG-N), histone substitution for isoforms (histone variants), depletion of one or the two H2A-H2B dimers (sub-nucleosomes), intimate two-particle association, or isomeric structural alterations. The resulting entities, some of them are only transient, acquire new properties useful for their specific roles in chromatin function. These structures are presented here in the chronological order of their identification, from the chromatosome to the sub-nucleosomal hexasome and tetrasome, and from the dinucleosomal altosome and nucleodisome to the nucleosome variants and altered forms: the old lexosome and the most recent reversome.