Figure 1. Structural elements of globular domains of linker and core histones. Each member of the five histone classes contains a globular region with a characteristic tertiary structure. The central domain of histone H1 consists of three -helices and a twisted antiparallel -structure. This structure was first derived from the globular domain of the H1 subtype H5 (Clark et al., 1993). It is very similar to the DNA-binding domain of transcription factors such as hepatic nuclear factor 3 (HNF3) and forkhead. The C-terminal globular portions of core histones contain characteristic histone-fold domains. These are contained in all four classes of core histones. They consist of a long central -helix flanked by two short helices: the long central helix provides the interface for dimerization. This histone-fold motif is also found in several transcription factors and associated proteins. The drawings are based on X-ray crystallography data of Clark et al. (1993) and Luger et al. (1997).

Figure 2. Histone gene organization in a main type and a replacement histone gene. Two histone H1 genes are taken as examples. Upper panel: a main type, replacement histone gene is characterized by two short untranslated regions (grey) flanking the uninterrupted (i.e. intronless) coding portion (blue). The sequence motif at the 3¢ end of the gene forms a tail-loop element at the mRNA level (inset). Together with a downstream purine-rich element it is the site of assembly of the 3¢ processing complex that creates the nonpolyadenylated tail of the replication-dependent histone mRNA. In contrast, the mRNA encoding the replacement histone H1° has long untranslated regions and the resulting mRNA is polyadenylated. The AATAAA motif in the gene (AAUAAA in the mRNA) is the processing signal for cleavage and polyadenylation. The H1° gene is an example for an intronless replacement histone gene; others contain introns (e.g. the H3.3A and B genes). The promoter region upstream of the transcribed region contains sequence motifs that are binding sites for general transcription factors, such as the TATA-, CCAAT- and GC-rich motifs. The CCAAT box is missing in the H1° promoter, but in both promoter types histone H1-specific sequence elements, such as the ‘H1 box’ AAACACA, have been identified.

Figure 3. Posttranslational modification sites within the N-terminal extended portions of the four core histones. The sites of acetylation at lysine (K) residues are shown in blue, phosphorylation of serine (S) residues in red and methylation at either lysine (K) or arginine (R) in green. Note that Lys-9 in H3 may be either acetylated or methylated. In addition to the modifications in the N-terminal region, the ubiqitin modification sites near the C-terminus of H2A and H2B histones are included (yellow boxes).