10 THE OPERON (Full Edition)
2 Regulation can be negative or positive
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- A repressor is a protein that
inhibits expression of a gene. It may act to prevent transcription by
binding to an operator site in DNA, or to prevent translation by
binding to RNA.
- The operator is the site on DNA at which a repressor protein binds to prevent transcription from initiating at the adjacent promoter.
- A transcription factor is required for RNA polymerase to initiate transcription at specific promoter(s), but is not itself part of the enzyme.
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In negative regulation a repressor protein binds to an operator to prevent a gene from being expressed.
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In
positive regulation a transcription factor is required to bind at the
promoter in order to enable RNA polymerase to initiate transcription.
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Figure 10.2
In negative control, a trans-acting repressor binds to the cis-acting operator to turn off transcription.
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A classic mode of control in bacteria is negative: a repressor
protein prevents a gene from being expressed. Figure 10.2
shows that the "default state" for such a gene is to be expressed via
the recognition of its promoter by RNA polymerase. Close to the
promoter is another cis-acting site called the operator,
which is the target for the repressor protein. When the repressor binds
to the operator, RNA polymerase is prevented from initiating
transcription, and gene expression is therefore turned off.
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Figure 10.3
In positive control, trans-acting factors must bind to cis-acting sites in order for RNA polymerase to initiate transcription at the promoter.
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An alternative mode of control is positive.
This is used in bacteria (probably) with about equal frequency to
negative control, and it is the most common mode of control in
eukaryotes. A transcription factor is required to assist RNA polymerase in initiating at the promoter.
Figure 10.3 shows that the typical default state of a eukaryotic gene is inactive:
RNA polymerase cannot by itself initiate transcription at the promoter.
Several trans-acting factors have target sites in the vicinity of the promoter, and binding
of some or all of these factors enables RNA polymerase to initiate transcription.
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The unifying theme is that regulatory proteins are trans-acting
factors that recognize cis-acting
elements (usually) upstream of the gene. The consequences of this
recognition are to activate or to repress the gene, depending on the
individual type of regulatory protein. A typical feature is that the
protein functions by recognizing a very short sequence in DNA, usually
<10 bp in length, although the protein actually binds over a
somewhat greater distance of DNA. The bacterial promoter is an example:
although RNA polymerase covers >70 bp of DNA at initiation, the
crucial sequences that it recognizes are the hexamers centered at – 35
and – 10.
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A significant difference in gene
organization between prokaryotes and eukaryotes is that structural
genes in bacteria are organized in clusters, while those in eukaryotes
occur individually. Clustering of structural genes allows them to be
coordinately controlled by means of interactions at a single promoter:
as a result of these interactions, the entire set of genes is either
transcribed or not transcribed. In this chapter, we discuss this mode
of control and its use by bacteria (for review see 82).
The means employed to coordinate control of dispersed eukaryotic genes are
discussed in Activating transcription.
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82 Miller, J. and Reznikoff, W.
(1978).
The Operon.
Cold Spring Harbor Symp. Quant. Biol..
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© Jones and Bartlett Publishers (2007)
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