Summary of Transcriptional Regulation

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Summary of Transcriptional Regulation
- transcription is the primary control point for
gene expression ? In all organisms but
particularly in bacteria - control is most often
effected by modulation of promoter
activity Promoters can be turned on and off by
binding regulatory proteins at sites near the
promoter on the DNA near in prokaryotes 100
200 b.p. in eukaryotes up to 50,000 b.p.
typically lt1000 b.p.
Positive Regulation Binding of regulatory
proteins increases transcription Negative
Regulation Binding of regulatory proteins
reduces transcription
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Operons

• - most commonly found in prokaryotes, with a few
  examples now known in eukaryotes.
• - in bacteria, when one promoter serves a series
  of clustered genes, the gene cluster is called an
  operon
• - all of these genes are transcribed into a
  single mRNA
• - each section of these mRNAs (called
  polycistronic mRNA) may then be translated
  independently
• - the genes in a given operon often encode for
  several enzymes active in a single metabolic
  pathway

gene a
b
c
promoter
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• The occurrence of genes in operons allows the
  expression of the genes to be controlled
  coordinately
• - a very economical situation (although
  translation rates of individual genes (cistrons)
  in the operon may vary

Figure 26.2
On
Off
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Induction Repression of Transcription
The expression (RNA production) of operons can be
induced or repressed by specific effector
molecules (e.g., chemicals, nutrients, etc.) ?
not all promoters in a cell are constantly
accessible to RNA polymerase ? which promoters
are accessible depends on what specific molecules
are present in the medium (and in the cell) ?
specific enzymes required for metabolism of
particular molecules can thus be induced or
repressed on demand
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Common Examples of Operons enzymes that convert
various sugars to glucose (a readily-metabolized
energy source)
Lac Operon Lactose Gal Operon Galactose Ara
Operon Arabinose
Induction Repression
lactose removed
Lac Operon
RNA
Concentration (normalized)
Enzyme
lactose added
Time
0
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Regulation of operons to optimize the use of
compounds to metabolize are not the only
examples. Many biosynthetic pathways are also
under this control.
Tryptophan (Trp) Operon synthesis of tryptophan
Trp suddenly available
No Trp
- when tryptophan becomes available to the cell
it shuts off all the machinery used to
synthesize the amino acid - why waste energy
making something that is freely available from
the environment?
Concentration (normalized)
Time
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Proteins Involved in Gene Control
I. Repressors Negatively acting regulatory
proteins, that bind to DNA AT or NEAR a promoter
site this binding prevents RNA polymerase from
associating with the promoter The repressor
binding site on DNA is called the Operator -A
specific DNA sequence recognized by the repressor
GENE
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Repressor proteins can combine with Effectors
(Small molecules), that greatly affect their
ability to bind their operator DNA. -look back
at allosteric proteins in your notes
2 Types of Effectors 1) Inducers Decrease
repressors binding affinity to operator
DNA (e.g., lactose binding to lac
repressor) 2) Co-Repressors Increase affinity -
repressor not active when co-repressor
absent (e.g., tryptophan for trp repressor)

DNA


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• II. Activators
• Positively acting regulatory proteins, that bind
  TO or NEAR a promoter site. This binding helps
  increase the frequency with which RNA polymerase
  binds to the promoter
• (best known examples are CAP or CRP protein)
• Two Types of activator protein
• Activator proteins that need an effector molecule
• Do not need an effector, but high levels of
  activator protein are needed for it to be
  effective

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Structure of the CRP-cAMP complex bound to DNA.
This protein is involved in the lac operon as we
will see along with several others in E. coli
DNA
CRP
cAMP
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Summary of Bacterial Gene Regulation

• A) Different genes/operons may be expressed at
  different levels according to how well their
  promoter matches an optimal promoter sequence
  (consensus sequence)
• B) The expression of genes and operons may also
  be modulated through various signals the main
  means of induction and repression of operons and
  individual genes include the use of
• Repressors - which inhibit transcription by
  binding to an operator DNA sequence near the
  promoter
• - which can be modulated by
• Effectors
• Co-repressors
• Activators - which enhance transcription by
  binding a DNA sequence near the promoter sequence
• - these may or may not be modulated by an effector

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(No Transcript)
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The Lac Operon
Figure 26.17

• Three structural genes in the operon z, y and a
• - these code for enzymes involved in lactose
  metabolism
• - these genes are expressed a very low levels all
  of the time
• - induced x1000 when lactose is present
• - also influenced by the level of glucose in the
  environment
• An adjacent gene (but not part of the lac
  operon), is lac i which codes the protein lac
  repressor

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• The lac repressor binds to its operator, which
  overlaps with the lac promoter
• ? Lactose binds to the repressor
• This results in a conformational change in the
  repressor, which disrupts in DNA binding domain
• Repressor no longer can bind its operator
  sequence

lactose
Lac operator DNA sequence
Active Repressor
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- the lac operator is 35 bp long - it is
downstream overlapping with the promoter - it
has a two-fold sequence symmetry (imperfect)
i.e., a palindromic sequence 22 bp out of 35 are
protected from nucleases during footprinting
experiments - the affinity of the repressor for
the operator sequence is 4x106 higher than for
other DNA sequences
Operator Sequence (spans region shown)
Figure 26.19 (Biochemistry)
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Structure of the Lac Repressor Bound to DNA
The crystal structure of the lac repressor of
Escherichia coli (LacR) at 2.6 A resolution. The
quaternary structure consists of two
dyad-symmetric dimers that are nearly parallel to
each other. This structure places all four DNA
binding domains of LacR on the same side of the
tetramer, and results in a deep, V-shaped cleft
between the two dimers. Each monomer contributes
a carboxyl-terminal helix to an antiparallel
four-helix bundle that functions as a
tetramerization domain.
- structure is consistent with it binding a
palindromic sequence (as with restriction enzymes)
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Figure 26.18
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The lac operon is also subject to POSITIVE
regulation, by CRP in the presence of cAMP

• The lac operon is induced by the presence lactose
  in the medium, but E. coli prefers to use glucose
  (better energy source)
• lac operon is also regulated by glucose levels
• glucose high ? Low transcription of lac
  operon
• glucose low ? High transcription of lac
  operon
• This correlates to the activity of CRP which
  activates lac operon in the presence of cAMP
  (cyclic AMP)
• glucose low ? cAMP high
• glucose high ? cAMP low

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Figure 26.21
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Levels of Control of Lac Operon Expression 3
Scenarios

• No Lactose around
• Operon switched off, no mRNA regardless of
  glucose
• Lactose present glucose also present
• The presence of lactose inactivates the repressor
  
• ? Transcription occurs
• Glucose present ? cAMP is low ? CRP does not
  help transcription
• Lactose present no glucose
• The presence of lactose inactivates the repressor
• ? Transcription occurs
• NO Glucose ? cAMP is high ? cAMP binds CRP
  (becomes activated) ? CRP binds Helps
  Transcription
• High Level of transcription

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The Trp Operon
- The Trp operon is expressed at high levels,
when tryptophan is scarce in a cell - When Trp is
abundant, it binds to the trp-repressor (normally
present in the nucleus) - Trp binding alters the
repressors conformation. The tryp-bound
repressor binds the trp operator blocks the 10
region from RNA polymerase ? Shuts Down trp
Operon. ? A case of feed-back inhibition, with
tryptophan acting as a Co-repressor.
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Figure 26.33 The Trp Operon