The N terminus

(A) General chromatin organization. Like other histone "tails," the N terminus of H3 (red) represents a highly conserved domain that is likely to be exposed or extend outwards from the chromatin fiber. A number of distinct post-translational modifications are known to occur at the N terminus f H3 including acetylation (green flag), phosphorylation (grey circle), and methylation (yellow hexagon). Other modifications are known and may also occur in the globular domain. (B) The N terminus of human H3 is shown in single-letter amino-acid code. For comparison, the N termini of human CENP-A, a centromere-specific H3 variant, and human H4, the nucleosomal partner to H3, are shown. Note the regular spacing of acetylatable lysines (red), and potential phosphorylation (blue) and methylation (purple) sites. The asterisk indicates the lysine residue in H3 that is known to be targeted for acetylation as well as for methylation; lysine 9 in CENP-A (bold) may also be chemically modified (see text). The above depictions of chromatin structure and H3 are schematic; no attempt has been made to accurately portray these structures.