2025/10/28 by Harsh Ranawat, Marc K M Cajili, Natalia Lopez‐Barbosa +3 · 1 voice · 5 citations
Biochemistry, Genetics and Molecular Biology · Agricultural and Biological Sciences · #Genomics and Chromatin Dynamics #Advanced Electron Microscopy Techniques and Applications #Chromosomal and Genetic Variations
paper · doi:10.1016/j.molcel.2025.10.001
Asgards are the closest archaeal relatives of eukaryotes, representing an important step in chromatin evolution. However, their chromatin organization has remained enigmatic until now. In this study, we present the first structures of Asgard chromatin assemblies formed by the Hodarchaeal histone HHoB. Our high-resolution cryo-electron microscopy (cryo-EM) structures reveal that this Asgard histone assembles into compact “closed” and into extended “open” hypernucleosomes. Thus, the closed hypernucleosome conformation is conserved across archaeal lineages, while the open conformation resembles a eukaryotic H3–H4 octasome and likely represents an Asgard-specific innovation. Moreover, we show that Mg²⁺ ions influence Asgard chromatin conformation, suggesting a regulatory role. Overall, our study provides the first structure-based model of Asgard chromatin organization, expanding our understanding of chromatin architecture in an evolutionary context. • Cryo-EM reveals chromatin assemblies formed by Asgard histone HHoB • HHoB histone forms both compact closed and extended open hypernucleosomes • Closed hypernucleosomes are conserved among archaea; open hypernucleosomes are Asgard-specific • Mg²⁺ ions regulate the balance between open and closed chromatin states Ranawat et al. show the cryo-EM structures of Asgard archaeal chromatin assemblies, revealing that the histone HHoB assembles into both compact closed and extended open hypernucleosomes. The closed conformation is conserved across archaea, while the open conformation represents an Asgard-specific innovation.