BLG 143 Study Guide - Final Guide: Membrane Transport Protein, Gene Expression, Escherichia Coli
27 Apr 2018
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Chapter 17 – Control of Gene Expression in Bacteria
➢ In bacteria, gene expression can be controlled at transcription, translation or
posttranslation
➢ Changes in gene expression allow bacterial cells to respond to environmental
changes
➢ Transcriptional control can be positive or negative
➢ Negative control occurs when a regulatory protein prevents transcription
➢ Positive control occurs when a regulatory protein increases the transcription rate
Cells don’t express all genes all of the time. They’d be very disorganized.
They’re selective about:
• Which genes are expressed
• How strongly genes are expressed
• When genes are expressed
Gene expression occurs when gene product (protein) is actively synthesized and used in
the cell.
Regulation of gene expression is critical to efficient of use of resources and thus survival.
17.1 Gene Regulation and Information Flow
Bacteria in body outnumbers the cells.
E.coli cells can use a variety of carbs to supply the carbon and energy they need. But as
daily diet changes, the availability of different sugars in your intestines varies.
Each type of nutrient requires a different membrane transport protein to bring the
molecule into the cell and a different suite of enzymes to process it.
Precise control of gene expression gives E.coli the ability to use the available sugars
efficiently.
Regulation Mechanisms
DNA mRNA protein activated protein
Gene expression can be controlled at any of these arrows.
The flow suggests:
1. The cell could avoid making the mRNAs for particular enzymes. If there’s no mRNA,
then ribosomes cannot make the gene product.
Transcriptional control occurs when regulatory proteins affect RNA polymerase’s
ability to bind to a promoter and initiate transcription.
2. If mRNA for an enzyme has been transcribed, the cell might have a way to prevent
mRNA from being translated into protein.
Translational control occurs when regulatory molecules alter the length of time an
mRNA survives before it’s degraded by ribonucleases, or affect translation initiation,
or affect elongation factors and other proteins during the translation process.
3. Some proteins are manufactured in an inactive form and have to be activated by
chemical modifications, such as the addition of a phosphate group
posttranslational control.
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All of the above occur in bacteria.
Transcriptional control efficient; saves the most energy for the cell since it stops the
process at the earliest possible point.
Translational control advantageous because it allows a cell to make rapid changes in the
relative amounts of different proteins.
Posttranslational control most rapid response to all of the 3 mechanisms.
There’s a trade-off between the speed of response and the conservation of ATP, amino acids
and other resources. Transcriptional is slow but efficient in resource us; posttranslational
is fast but energetically expensive.
Bacteria possess translational and posttranslational controls and some genes, like those
that code for the enzymes required for glycolysis, are transcribed all the
time/constitutively.
Gene expression is not an all-or-none proposition.
Genes are not just on or off, the level of expression is highly variable.
Variation in gene expression allows cells to respond to changes in their environment.
Metabolizing Lactose – A Model System
➢ Monod and Jacob introduced lactose metabolism in E. coli as a model system
➢ E. coli uses a variety of sugars for ATP production
➢ Sugars also serve as raw material in the synthesis of amino acids, vitamins and other
complex compounds; glucose is E. coli’s preferred carbon source
➢ Lactose, sugar found in milk, is also used by E. coli only when glucose supplies are
limited
➢ Lactose is a disaccharide made up one glucose and one galactose
➢ To use lactose, E. coli first has to transport the sugar into the cell
➢ Once lactose is inside the cell, enzyme β-galactosidase catalyzes a reaction that
breaks down the sugar into glucose and galactose
➢ The glucose released goes directly into the glycolytic pathway
➢ Other enzymes convert the galactose to a substance that can also be processed in
the glycolytic pathway
(preference for glucose over lactose makes sense since making pyruvates from glucose
takes fewer steps than making pyruvates from lactose)
17.2 Identifying Genes under Regulatory Control
To find mutants that’re associated with a particular trait:
1. Generate large number of individuals with mutations at random locations in their
genomes. Exposed E. coli populations to X-rays, UV light or mutagens, chemicals that
damage DNA and increase mutation rates
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Document Summary
Chapter 17 control of gene expression in bacteria posttranslation. In bacteria, gene expression can be controlled at transcription, translation or. Changes in gene expression allow bacterial cells to respond to environmental changes. Transcriptional control can be positive or negative. Negative control occurs when a regulatory protein prevents transcription. Positive control occurs when a regulatory protein increases the transcription rate. Cells don"t express all genes all of the time. They"re selective about: which genes are expressed, how strongly genes are expressed, when genes are expressed. Gene expression occurs when gene product (protein) is actively synthesized and used in the cell. Regulation of gene expression is critical to efficient of use of resources and thus survival. E. coli cells can use a variety of carbs to supply the carbon and energy they need. But as daily diet changes, the availability of different sugars in your intestines varies.
