Histones are DNA-binding proteins that play a crucial role in DNA packaging and gene regulation in eukaryotes and archaea. In eukaryotes, histones form octamers that constitute the core of the nucleosome, the fundamental unit of chromatin. Archaeal histones, on the other hand, form tetramers that assemble into extended superhelices upon DNA binding. Although previously thought to occur only in archaea and eukaryotes, histone homologs have recently been discovered in bacteria. This work presents the dimeric crystal structure of the bacterial histone HBb fromBdellovibrio bacteriovorusdetermined at a resolution of 1.06 Å, representing the first-ever structure of any histone protein determined at atomic resolution. Furthermore, this study shows that HBb binds DNA and is essential for bacterial viability, suggesting that bacterial histone homologs likely have a similar biological function as their eukaryotic and archaeal counterparts. These findings have important implications for our understanding of the fundamental processes of DNA organization and regulation in all domains of life.
