11 REGULATORY CIRCUITS (Full Edition)
10 Translation can be regulated
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- A repressor protein can regulate
translation by preventing a ribosome from binding to an initiation codon.
- Accessibility of initiation codons
in a polycistronic mRNA can be controlled by changes in the structure of the mRNA
that occur as the result of translation.
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Translational control is a notable feature of operons coding for components of the
protein synthetic apparatus. The operon provides an arrangement for coordinate
regulation of a group of structural genes. But, superimposed on it, further controls,
such as those at the level of translation, may create differences in the
extent to which individual genes are expressed.
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Figure 11.14
A regulator protein may block translation by binding to a site on mRNA that overlaps
the ribosome-binding site at the initiation codon.
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>A similar type of mechanism is used to achieve translational control in several
systems. Repressor function is provided by a protein that binds to a target region
on mRNA to prevent ribosomes from recognizing the initiation region. Formally
this is equivalent to a repressor protein binding to DNA to prevent RNA polymerase
from utilizing a promoter. Figure 11.14 illustrates the most common form of this
interaction, in which the regulator protein binds directly to a sequence that includes
the AUG initiation codon, thereby preventing the ribosome from binding.
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Figure 11.15
Proteins that bind to sequences within the initiation regions of mRNAs may function
as translational repressors.
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Some examples of translational repressors and their targets are summarized in Figure
11.15. A classic example is the coat protein of the RNA phage R17; it binds to a
hairpin that encompasses the ribosome binding site in the phage mRNA. Similarly
the T4 RegA protein binds to a consensus sequence that includes the AUG initiation
codon in several T4 early mRNAs; and T4 DNA polymerase binds to a sequence in its
own mRNA that includes the Shine-Dalgarno element needed for ribosome binding.
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Figure 11.16
Secondary structure can control initiation. Only one initiation site is available
in the RNA phage, but translation of the first cistron changes the conformation
of the RNA so that other initiation site(s) become available.
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Another form of translational control occurs when translation of one cistron requires
changes in secondary structure that depend on translation of a preceding cistron.
This happens during translation of the RNA phages, whose cistrons always are expressed
in a set order. Figure 11.16 shows that the phage RNA takes up a secondary structure
in which only one initiation sequence is accessible; the second cannot be recognized
by ribosomes because it is base paired with other regions of the RNA. However, translation
of the first cistron disrupts the secondary structure, allowing ribosomes to bind
to the initiation site of the next cistron. In this mRNA, secondary structure controls
translatability.
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©Jones and Bartlett Publishers (2007)
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