The Expanding Histone Universe: Histone-Based DNA Organization in Noneukaryotic Organisms

Histones are small basic proteins that form the proteinaceous core of the nucleosome, the repeating building block of chromatin in all eukaryotes. Long thought to be exclusive to eukaryotes, histones are now increasingly appreciated for their roles in organizing genomes across all domains of life, namely in archaea, bacteria, and even viruses. We survey recent advances in our understanding of the imaginative uses of histones in disparate biological entities, ranging from nucleosome-like metastable particles in giant viruses to slinky-like hypernucleosomes in archaea to bacterial histones that bind DNA in decidedly unorthodox ways. Across these different contexts, we examine how DNA compaction and conformation emanate from evolutionarily conserved aspects of histone structure, including how the oligomeric states of histones dictate their capacity to contort DNA in different conformations. It appears that relatively small tweaks to the amino acid sequences of histones can result in structural and functional variations in DNA binding. As such, nucleosomes in eukaryotes sample only a narrow range of possible structures.