Chromatin organization in Asgard archaea: histones, SMC complexes, and the archaeal roots of eukaryotic chromatin

The genomes of organisms across the tree of life are structurally and functionally organized into chromatin. In eukaryotes, within an organelle called the nucleus, chromatin is shaped by histones and structural maintenance of chromosomes (SMC) complexes, among other factors. The closest prokaryotic relatives of eukaryotes, the Asgard archaea, lack a nucleus, but their genomes encode multiple histones and SMC complexes. Understanding chromatin organization in Asgard archaea is key to understanding how eukaryotic chromatin evolved. However, to date, experimental information on the mechanisms of action of these proteins is largely lacking, particularly in vivo. In this review, we discuss the remarkable diversity of Asgard archaeal histones, the characteristics of SMC complexes, and their potential structural and regulatory roles in Asgard archaea.