Archaeal nucleosomes.

Archaea contain histones that have primary sequences in common with eukaryal nucleosome core histones and a three-dimensional structure that is essentially only the histone fold. Here we report the results of experiments that document that archaeal histones compact DNA in vivo into structures similar to the structure formed by the histone (H3+H4)2 tetramer at the center of the eukaryal nucleosome. After formaldehyde cross-linking in vivo, these archaeal nucleosomes have been isolated from Methanobacterium thermoautotrophicum and Methanothermus fervidus, visualized by electron microscopy on plasmid and genomic DNAs, and shown by immunogold labeling, SDS/PAGE, and immunoblotting to contain archaeal histones, cross-linked into tetramers. Archaeal nucleosomes protect approximately 60 bp of DNA and multiples of approximately 60 bp from micrococcal nuclease digestion, and immunoprecipitation has demonstrated that most, but not all, M. fervidus genomic DNA sequences are associated in vivo with archaeal histones.