ISSN 1022-7954, Russian Journal of Genetics, 2006, Vol. 42, No. 9, pp. 970–984. © Pleiades Publishing, Inc., 2006.
Original Russian Text © D.E. Koryakov, 2006, published in Genetika, 2006, Vol. 42, No. 9, pp. 1170–1185.
Almost no free DNA occurs in eukaryotic cell
nuclei. DNA is associated with many proteins to form a
complex known as chromatin. The association of DNA
with proteins is not random, but rather shows a strongly
hierarchic multilevel organization wherein nucleosomes
correspond to the ﬁrst level. Nucleosomes are repetitive
units that each represent a protein globule with DNA
wrapping round it by 1.75 turns [1–3] (Fig. 1). A globule
includes histones of ﬁve types: H1, H2A, H2B, H3, and
H4, which are all enriched in basic amino acid residues,
lysine and arginine. H2A, H2B, H3, and H4 molecules
form a wedge-shaped core octamer: its narrow part is
formed by an (H3–H4)
tetramer, and the wide part
consists of two H2A–H2B dimers [4–6]. The C-termi-
nal regions of the octamer histones are tightly folded,
while the ﬂexible N-terminal regions, also known as
N tails, are unfolded and freely radiate outwards [7, 8].
One molecule of histone H1 binds to the outer surface
of the nucleosome in the region of the (H3–H4)
ramer, thereby ﬁxing DNA on it [9–13]. Histone H1 has
a globular central region of approximately 80 amino
acid residues and free C and N ends . In bird and
reptilian erythrocytes, histone H1 is replaced by its
variant known as H5 in inactive chromatin . The
DNA region corresponding to one nucleosome varies in
size, reaching 200 bp [16, 17]. Of these, 146 bp are
directly associated with the histone octamer, and the
other several tens of base pairs link two neighbor
nucleosomes [5, 13, 18, 19].
The histone sequences are highly conserved. The
earliest sequencing data showed that histone H4 differs
only in two residues between calf and pea [20, 21]. The
histone sequences are now known for many organisms,
from protozoa to humans . Sequence comparisons
demonstrated that histones H3 and H4 are conserved to
the greatest extent, while histones H2A and H2B are
least conserved .
A detailed study of the sequences showed that his-
tones of every type but H4 are heterogeneous in cells of
one organism, forming a family. Each family includes a
major type and minor fractions known as “histone vari-
ants.” Different organisms possess up to four variants of
histone H3, approximately eight variants of histone
H2A, up to seven variants of histone H1, and at least
three variants of histone H2B. Most, if not all, variants
are associated with a certain process, e.g., transcription
or X-chromosome inactivation [24–27].
Every process occurring in chromatin is a chain of
enzymatic reactions and essentially concerns transmis-
sion, protection, or realization of genetic information.
Such processes can be grouped by stability and period-
Histone Modification and Regulation
of Chromatin Function
D. E. Koryakov
Institute of Cytology and Genetics, Russian Academy of Sciences, Novosibirsk, 630090 Russia;
fax: (383) 330-16-65; e-mail: firstname.lastname@example.org
Department of Cytology and Genetics, Novosibirsk State University, Novosibirsk, 630090 Russia
Received March 2, 2006
—Nucleosomes play two main roles, acting as basal units in DNA compaction and coordinating most
processes in chromosomes. The coordination is due to modiﬁcation of histones, proteins forming nucleosomes.
The review brieﬂy describes the nucleosome structure and major modiﬁcations of histones and considers the
role of such modiﬁcations in transcriptional suppression and activation.
Organization of an individual nucleosome. Different
histones are differently shadowed. See text for comments.