Practice question

Histones were long considered exclusive to eukaryotes, organizing DNA into nucleosomes with wrapping around octameric cores. Discovery of archaeal histones revealed evolutionary connection, yet distribution is not uniform. Euryarchaeota such as Methanothermus fervidus, Methanothermus sociabilis, and halophiles like Halobacterium salinarum encode HMf and HMt family histones that form tetramers wrapping DNA into nucleosome-like particles and regulating transcription by restricting access. These histones share histone fold motif with eukaryotic H3 and H4. In contrast, most members of Crenarchaeota, particularly hyperthermophilic genera like Sulfolobus acidocaldarius, Thermoproteus, and Pyrolobus, lack sequence homologs of true histones and instead use alternative DNA packaging proteins such as Alba, Sul7d, and Cren7 that coat DNA and introduce supercoiling. This pattern suggests histone-based chromatin is characteristic of Euryarchaeota rather than all archaea, reflecting diverse strategies for genome compaction under extreme conditions and differing sensitivity to thermal denaturation, influencing gene regulation and adaptation. Alba proteins abundant in Sulfolobus bind cooperatively to double-stranded DNA, bridging strands, regulating transcription and protecting against thermal denaturation, illustrating how crenarchaeota employ non-histone architectural proteins functionally analogous to histones yet structurally unrelated, diversifying chromatin strategies across archaeal phylum.