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Summary chapter 14
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Summary of 11 pages for the course Genetics at VU (chapter 14)
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CHAPTER 14Gene regulation means that the level of gene expression can vary
under different conditions. Gene regulation is important for cellular
processes.
- Genes that are unregulated are termed constitutive; they are
constantly expressed.
They encode proteins that are continuously necessary for the survival
of the cell.
The benefit of regulating genes is that encoded proteins/RNAs will be
produced only when required.
14.1 TRANSCRIPTIONAL REGULATION
The most common way to regulate gene expression in bacteria is by
influencing the rate at which transcription is initiated.
Transcriptional regulation involves the actions of two main types of
regulatory proteins:
• Repressors Bind directly or indirectly to DNA and inhibit
transcription: Negative control
• Activators Bind directly or indirectly to DNA and increase
transcription: Positive control
Small effector molecules (ligands) affect transcription regulation: short
peptides, metabolites, drugs, inorganic compounds…
These bind to regulatory proteins and not to DNA directly
The presence of a small effector molecule may increase transcription – inducers. They
function in two ways: Genes that are regulated in this manner are termed inducible.
- Bind activators and cause them to bind to DNA
- Bind repressors and prevent them from binding to DNA
The presence of a small effector molecule may decrease transcription. Genes that are
regulated in this manner are termed repressible.
- Corepressors bind to repressors and cause them to bind to DNA
- Inhibitors bind to activators and prevent them from binding to DNA
INDUCER MOLECULE
Regulatory proteins have two binding sites
- One for a small effector molecule
- The other for DNA
1
CAROLINA SANZ HERNÁNDEZ
, INHIBITORY MOLECULE
14.2 REGULATION OF THE lac OPERON
The phenomenon of enzyme adaptation is due to the synthesis of cellular proteins
At the turn of the 20th century, scientists made the following observation:
A particular enzyme appears in the cell only after the cell has been exposed to the
enzyme’s substrate. This observation became known as enzyme adaptation. (When a
bacterium is not exposed to a particular substance, it does not make the enzyme needed to
metabolize that substance).
François Jacob and Jacques Monod at the Pasteur Institute in Paris were interested in this
phenomenon. They focused their attention on lactose metabolism in E. coli.
Operon concept
An operon is a regulatory unit consisting of a few structural genes under the control of
one promoter and one terminator.
It encodes a polycistronic (poly-ORF) mRNA that contains the coding sequence for two or more
structural proteins. This allows a bacterium to coordinately regulate a group of genes that
encode proteins involved in the same process / function.
An operon contains several different regions: Promoter: operator; terminator; structural
genes.
(Not all genes in bacteria are in operons. Operons don’t have terminator nor
promoters in between).
Inducible vs Repressible Regulation
Operons involved in catabolism are typically inducible (lac Operon). The substance to be
broken down acts as the inducer.
Operons involved in anabolism are repressible (trp Operon). The inhibitor or corepressor is
the small molecule that is the product of the operon
Transcriptional units
Lac OPERON
a. DNA elements: Involved in transcriptional regulation.
- ‘Promoter’ (lacP) Bound by RNA polymerase
- Operator (lacO) Bound by the lac repressor protein
- CAP site recognized by an activator protein- catabolite activator protein (CAP).
b. Structural genes: Contain the coding sequence for the enzymes.
- lacZ Encodes b-galactosidase
- Enzymatically cleaves lactose and lactose analogues
- Also converts lactose into allolactose (an isomer)
- lacY Encodes lactose permease
- Membrane protein required for transport of lactose and analogues
- lacA Encodes transacetylase
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CAROLINA SANZ HERNÁNDEZ
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