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

1164 Chapter 21: Development of Multicellular Organisms
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Abdominal A
Abdominal B
Chromosome 3
underlying these phenomena are still not well understood, but their consequences
are profound. We shall see that the serial organization of gene expression
in the Hox complex is a fundamental feature that has been highly conserved in the
course of animal evolution.
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complex
Bithorax
complex
Figure 21–24 The patterns of
expression compared to the
chromosomal locations of the
genes of the Hox complex. The
diagram shows the sequence of
genes in each of the two subdivisions
of the chromosomal complex. This
corresponds, with minor deviations,
to the spatial sequence in which
the genes are expressed, shown
in the photograph of a Drosophila
embryo at the so-called germ band
retraction stage, about 10 hours after
fertilization. The embryo has been
stained by in situ hybridization with
differently labeled probes to detect
the mRNA products of different Hox
genes in different colors. (Photograph
courtesy of William McGinnis, adapted
from D. Kosman et al., Science
305:846, 2004. With permission from
AAAS.)
MBoC6 m22.44/22.24
Trithorax and Polycomb Group Proteins Enable the Hox
Complexes to Maintain a Permanent Record of Positional
Information
The spatial pattern of expression of the genes in the Hox complex is set up by signals
acting early in development, but the consequences are long lasting. Although
the pattern of expression undergoes complex adjustments as development proceeds,
the Hox complexes serve to stamp each cell and its progeny with a permanent
record of the A-P position that the cell occupied in the early embryo. In this
way, the cells of each segment are equipped with a long-term memory of their
location along the A-P axis of the body. This memory trace is somehow imprinted
on the Hox complexes, and it governs the segment-specific identity not only of the
larval segments, but also of the structures of the adult fly.
The molecular mechanism of this memory of positional information relies on
two types of regulation. One is from the Hox genes themselves: many of the Hox
proteins autoactivate the transcription of their own genes, thereby helping to keep
the genes on indefinitely. Another crucial input is from two large, complementary
sets of proteins, called the Trithorax group and the Polycomb group, which
stamp the chromatin of the Hox complex with a heritable record of its embryonic
state of activation or repression. These are key general regulators of chromatin
structure that can be shown to be critical for cell memory: if genes of the Trithorax
or Polycomb group are defective, the pattern of expression of the Hox genes is set
up correctly at first, but it is not correctly maintained as the embryo grows older.
The two sets of regulators act in opposite ways. Trithorax group proteins are
needed to maintain the transcription of Hox genes in cells where transcription has
already been switched on. In contrast, Polycomb group proteins form stable complexes
that bind to the chromatin of the Hox complex and maintain the repressed
state in cells where Hox genes have not been activated at the critical time (Figure
21–25). How such changes in chromatin can store developmental cell memory is
discussed in Chapters 4 and 7.
The D-V Signaling Genes Create a Gradient of the Transcription
Regulator Dorsal
As with the patterning along the Drosophila A-P axis just discussed, the patterning
along the dorsoventral (D-V) axis begins with maternal gene products that define