Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

192 Chapter 4: DNA, Chromosomes, and Genomes
ATP-dependent
chromatin
remodeling
complex
histone chaperone
ATP
ADP
EXCHANGE OF
H2A–H2B DIMERS
Figure 4–27 Nucleosome removal and histone
exchange catalyzed by ATP-dependent chromatin
remodeling complexes. By cooperating with specific
members of a large family of different histone chaperones,
some chromatin remodeling complexes can remove
the H2A–H2B dimers from a nucleosome (top series of
reactions) and replace them with dimers that contain a
variant histone, such as the H2AZ–H2B dimer (see Figure
4–35). Other remodeling complexes are attracted to
specific sites on chromatin and cooperate with histone
chaperones to remove the histone octamer completely
and/or to replace it with a different nucleosome core
(bottom series of reactions). Highly simplified views of the
processes are illustrated here.
ATP ADP ATP ADP
histone
chaperone
DNA lacking
nucleosome
EXCHANGE OF
NUCLEOSOME CORE
(HISTONE OCTAMER)
How nucleosomes are organized into condensed arrays is unclear. The structure
of a tetranucleosome (a complex of four nucleosomes) obtained by x-ray
crystallography and high-resolution electron microscopy of reconstituted chromatin
have been used to support a zigzag model for the stacking of nucleosomes
in a 30-nm fiber (Figure 4–28). But cryoelectron microscopy of carefully prepared
nuclei suggests that most regions of chromatin are less regularly structured.
What causes nucleosomes to stack so tightly on each other? Nucleosome-tonucleosome
linkages that involve histone tails, most notably the H4 tail, constitute
one important factor (Figure 4–29). Another important factor is an additional
MBoC6 m4.30/4.26
histone that is often present in a 1-to-1 ratio with nucleosome cores, known as
histone H1. This so-called linker histone is larger than the individual core histones
and it has been considerably less well conserved during evolution. A single histone
H1 molecule binds to each nucleosome, contacting both DNA and protein,
and changing the path of the DNA as it exits from the nucleosome. This change in
the exit path of DNA is thought to help compact nucleosomal DNA (Figure 4–30).
Figure 4–28 A zigzag model for the 30-
nm chromatin fiber. (A) The conformation
of two of the four nucleosomes in a
tetranucleosome, from a structure
determined by x-ray crystallography.
(B) Schematic of the entire tetranucleosome;
the fourth nucleosome is not visible, being
stacked on the bottom nucleosome and
behind it in this diagram. (C) Diagrammatic
illustration of a possible zigzag structure
that could account for the 30-nm chromatin
fiber. (A, PDB code: 1ZBB; C, adapted
from C.L. Woodcock, Nat. Struct. Mol. Biol.
12:639–640, 2005. With permission from
Macmillan Publishers Ltd.)
(B)
(C)
(A)