ABSTRACT
Histones are small, basic, abundant DNA-binding proteins that form the protein core around which chromosomal DNA is wrapped, by electrostatic interactions, to generate nucleosomes (Wolffe, 1992). Histone-containing nucleoprotein complexes are present in both Archaea (Pereira et a/., 1997) and Eukarya, consistent with their divergence from a common histone-containing ancestral structure that existed before the separation of the archaeal and eukaryal lineages. Histones may have originated in hyperthermopbiles as a means to protect their chromosomes from thermal denaturation. All histones contain the histone fold structural motif, namely three a.-helices separated by 13-strand regions that are stabilised by protein dimerisation. Archaeal histones are essentially dimers of histone fold motifs; eukaryal histones are dimers of histone folds with short N-terminal and Cterminal amino acid extensions; and histone folds also exist as domains within more complex proteins. In this chapter we review the structure of the histone fold, and use a structure-based alignment to discuss the evolution and divergence of histone foldcontaining proteins. Bacteria also contain several different abundant, small, positively charged proteins, often termed 'histone-like' proteins (Schmid, 1990). These proteins do not, however, have primary sequences in common with histones, they do not contain histone fold motifs, and the complexes that they form with DNA are unrelated to nucleosomes.
