Core histones of the amitochondriate protist, Giardia lamblia

Genes coding for the core histones H2a, H2b, H3, and H4 of Giardia lamblia were sequenced. A conserved organism- and gene-specific element, GRGCGCAGAT TTVGG, was found upstream of the coding region in all core histone genes. T he derived amino acid sequences of all four histones were similar to their homologs in other eukaryotes, although they were among the most divergent m embers of this protein family. Comparative protein structure modeling combi ned with energy evaluation of the resulting models indicated that the G. la mblia core histones individually and together can assume the same three-dim ensional structures that were established by Xray crystallography for Xenop us laevis histones and the nucleosome core particle. Since G, lamblia repre sents one of the earliest-diverging eukaryotes in many different molecular trees, the structure of its histones is potentially of relevance to underst anding histone evolution. The G. lamblia proteins do not represent an inter mediate stage between archaeal and eukaryotic histones.