PARALLEL ORIGINS OF THE NUCLEOSOME CORE AND EUKARYOTIC TRANSCRIPTION FROM ARCHAEA

Computational sequence analysis of 10 available archaean histone-like proteins has shown that this family is not only divergently related to the eukaryotic core histones H2A/B, H3, and H4, but also to the centr al domain of subunits A and C of the CCAAT-binding factor (CBF), a tra nscription factor associated with eukaryotic promoters. Despite the lo w sequence identity, it is unambiguously shown that the core histone f old shares a common evolutionary history. Archaean histones and the tw o CBF families show a remarkable variability in contrast to eukaryotic core histones. Conserved residues shared between families are identif ied, possibly being responsible for the functional versatility of the core histone fold. The H4 subfamily is most similar to archaean protei ns and may be the progenitor of the other core histones in eukaryotes. While it is not clear whether archaean histones are more actively inv olved in transcription regulation, the present observations link two p rocesses, nucleosomal packing and transcription in a unique way. Both these processes, evidently hybrid in Archaea, have originiated before the ermergence of the eukaryotic cell.