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

428 Chapter 7: Control of Gene Expression
Figure 7–72 Visualization of P-bodies.
Human cells were stained with antibodies
to a component of the mRNA decapping
enzyme Dcp1a (left panels) and to the
Argonaute protein (middle panels). As
described later in this chapter, Argonaute
is a key component of RNA interference
pathways. The merged image (right panels)
shows that the two proteins co-localize to
P-bodies in the cytoplasm. (Adapted from
J. Liu et al., Nat. Cell Biol. 7:719–723,
2005. With permission from Macmillan
Publishers Ltd.)
20 µm
section—including decapping and RNA degradation—take place in aggregates
known as Processing- or P-bodies, which are present in the cytosol (Figure 7–72).
Although many mRNAs are eventually degraded in P-bodies, some remain
intact and are later returned to the pool of translating mRNAs. To be “rescued”
in this way, mRNAs move from P-bodies to another type of aggregate known as a
stress granule, which contains translation initiation factors, poly-A-binding protein,
and small ribosomal subunits. Translation itself does not occur in stress
granules, but mRNAs can become “translation-ready” as the proteins bound to
them in P-bodies are replaced with those in stress granules. The movement of
mRNAs between active translation, P-bodies, and stress granules can be seen as
an mRNA cycle (Figure 7–73) where MBoC6 the m7.114/7.73 competition between translation and
mRNA degradation is carefully controlled. Thus, when translation initiation is
blocked (by starvation, drugs, or genetic manipulation), stress granules enlarge
as more and more nontranslated mRNAs are moved directly into them for storage.
Clearly, once a cell has made the large investment in producing a properly processed
mRNA molecule, it carefully controls its subsequent fate.
Summary
Many steps in the pathway from RNA to protein are regulated by cells in order to
control gene expression. Most genes are regulated at multiple levels, in addition
to being controlled at the initiation stage of transcription. The regulatory mechanisms
include (1) attenuation of the RNA transcript by its premature termination,
(2) alternative RNA splice-site selection, (3) control of 3ʹ-end formation by cleavage
and poly-A addition, (4) RNA editing, (5) control of transport from the nucleus to
the cytosol, (6) localization of mRNAs to particular parts of the cell, (7) control of
translation initiation, and (8) regulated mRNA degradation. Most of these control
processes require the recognition of specific sequences or structures in the RNA molecule
being regulated, a task performed by either regulatory proteins or regulatory
RNA molecules.
NUCLEUS
CYTOSOL
mRNA
AAA
P-body
translation
stress granule
Figure 7–73 Possible fates of an mRNA
molecule. An mRNA molecule released
from the nucleus can be actively translated
(center), stored in stress granules (right), or
degraded in P-bodies (left). As the needs
of the cell change, mRNAs can be shuffled
from one pool to the next, as indicated by
the arrows.