Regulation of gene and cellular
activity
Regulating Gene Expression
• Cells use different mechanisms to sense and
respond to conditions within or outside the cell.
• Regulatory proteins help a cell sense internal
changes and alter its gene expression to match.
Figure 10.1A
Figure 10.1B and C
Figure 10.3
Regulating Gene Expression
• Microbes control gene expression at several
levels:
•
•
•
•
•
- Alterations of DNA sequence
- Control of transcription
- Control of mRNA stability
- Translational control
- Posttranslational control
The E. coli Lactose Operon
• The lactose utilization lacZYA operon of E. coli was the
first gene regulatory system described.
• First, here’s how lactose is transported and metabolized.
Figure 10.6
The Organization of the LacZYA Operon
Figure 10.5A
Scenario 1: Absence of Lactose
• LacI binds as a tetramer to the operator region.
•
- It represses the lac operon by preventing open
complex formation by RNA polymerase.
Figure 10.5B
Scenario 2: Presence of Lactose
• b-galactosidase (LacZ), when at low concentrations,
cleaves and rearranges lactose to make the inducer
allolactose.
• Allolactose binds to LacI, reducing its affinity to the
operator and thus allowing induction of the operon.
Figure 10.5C
Activation of the lac Operon by cAMP-CRP
• Maximum expression of the lac operon requires the presence of
cAMP and cAMP receptor protein (CRP).
•
- The cAMP-CRP complex binds to the promoter.
•
- Interacts with RNA pol to increase the rate of
transcription initiation
Figure 10.8
Catabolite Repression
• In catabolite repression, an
operon enabling the
catabolism of one nutrient is
repressed by the presence of
a more favorable nutrient
(commonly glucose).
The biphasic curve of a
culture growing on two
carbon sources is often
called diauxic growth.
Figure 10.10
Catabolite Repression
• Glucose transport by the
phosphotransferase system
causes catabolite
repression by inhibiting the
LacY permease activity.
•
- This is termed
inducer exclusion.
Figure 10.11
The E. coli Lactose Operon
• Animation: The lac operon
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Figure 10.15
Attenuation of the trp Operon
• Attenuation is a regulatory mechanism in which
translation of a leader peptide affects transcription of
a downstream structural gene.
The attenuator region of
the trp operon has 2
trp codons and is
capable of forming
stem-loop structures.
Figure 10.16A
The Transcriptional Attenuation Mechanism
of the trp Operon
Figure 10.16B and C
Figure 10.18
Figure 10.24
Figure 10.26A
Figure 10.28
Quorum Sensing
• Quorum sensing refers to the process where
bacterial cells work together at high density.
- It was discovered in
Vibrio fischeri, a
bioluminescent
bacterium that
colonizes the light
organ of the Hawaiian
squid.
Figure 10.30ACD
Figure 10.31
Transcriptional Attenuation
• Animation: Transcriptional Attenuation
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Chemotaxis: Molecular Events
• Animation: Chemotaxis: Molecular Events
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Quorum Sensing
• Animation: Quorum Sensing
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