Archaeal histone HTkC hypercompacts DNA

Histones are important organizers of chromatin in eukaryotes and archaea. In eukaryotes, the core histones assemble with DNA to form the octameric nucleosome. In archaea, histones form hypernucleosomes that are not restricted to an octameric histone core but can extend to variable lengths. We previously identified face-to-face (FtF) histones as a widely distributed group of archaeal histones that assemble into toroidal tetramer structures, distinct from nucleosomal histones. Here, we characterize the FtF histone HTkC from Thermococcus kodakarensis, which also encodes the canonical histones HTkA and HTkB. We show that HTkC wraps DNA around its toroidal tetramer and forms highly compact nucleoprotein complexes, achieving a level of compaction approximately twice that of hypernucleosomes. Consistent with a major chromatin-organizing role, htkC is among the most highly expressed genes in T. kodakarensis and its deletion leads to impaired growth. Together, these findings establish FtF histones as important organizers of archaeal chromatin alongside classical histones.