Gene Regulation 1 - Regulation of Gene Activity in Prokaryotes Flashcards
Lecture Outcomes
- List differences between gene regulation in prokaryotes and eukaryotes
- Understand the effect of and reasons for gene regulation
- List four ways that a cell can control the proteins it makes
- Describe the basic principles of coordinate regulation, catabolic vs. anabolic pathways,
and positive vs. negative regulation - Describe the tryptophan operon of E. coli and its negative regulation, using diagrams
- Define the terms operon, promoter, operator, repressor and polycistronic mRNA
Living cells need to make a lot of
decisions all the time!
Genes may be:
* Constitutive genes: always
expressed as are essential for the
basics of life e.g. DNA synthesis,
replication and repair, RNA and
protein synthesis
* Contingency genes: encode
products that confer an advantage
under special conditions e.g. heat
stress, pH stress, starvation,
carbon source availability
Every operation in a living cell must be co-ordinately regulated
Gene regulation: differences between
prokaryotes and eukaryotes
regularotyr decisions photo
enzymes photo
Regulation of gene activity in prokaryotes
The effect: The number of protein molecules produced per unit time by active genes varies from gene to gene and
varies in response to the environment.
The reason: To satisfy the needs of the cell but to avoid wasteful synthesis, e.g:
* Molecules needed occasionally are synthesized only when needed
* An enzyme that consumes the substrate of a 2nd enzyme is usually inhibited if end-product of 2nd enzyme
is required:
- If there are 2 pathways for energy production the cell will ‘choose’ the one that yields the most energy
The mechanisms:
* There are multiple ways used to regulate gene expression, both transcription and translation can be regulated.
* A common principle is on/off regulation: a system is turned on when needed, off when not needed
Four ways a cell can control the amount
of proteins it makes
- Transcriptional control: controlling when and how often a given gene is transcribed
- RNA processing control: controlling how the RNA transcript is spliced or otherwise
processed - Translational control: selecting which mRNAs in the cytoplasm are translated by ribosomes
- Post-translational control: e.g. protein activity control: selectively activating or inactivating
proteins after they have been made
For prokaryotes, the initiation of transcription
is the most important point of control
Principles of regulation
Principles of regulation
3.3. Negative versus positive regulation: overview
3.3. Negative versus positive regulation: overview
Negative regulation:
A REPRESSOR protein is present in the cell and prevents transcription. In some cases, an antagonist of the
repressor (INDUCER) is needed to remove the repressor from the DNA and allow initiation of transcription.
In other cases, a COREPRESSOR binds to the repressor and allows the repressor to block transcription.
However, in all cases, when the repressor is bound to the DNA, it prevents transcription of the gene.
3.3. Negative versus positive regulation: overview
3.3. Negative versus positive regulation: overview
Positive regulation:
An ACTIVATOR binds to the DNA and allows transcription. There is no repressor. However, a small
molecule LIGAND (such as sugars, amino acids, salts, metals etc.) may bind the activator. This ligand
determines whether the activator binds the DNA. However, in all cases, when the activator binds to the
DNA, it results in activation of transcription of the gene.
- Negative versus Positive regulation
Genetic organisation of bacterial
chromosomes
Gene:
* DNA sequence that codes for a polypeptide, tRNA or rRNA
* Represented as arrows on genetic map
* Found on both strands of dsDNA
* Generally do not overlap
e.g. Escherichia coli contains a single circular chromosome ~ 4.6 X
106 base pairs (bp) ~ 4300 proteins
* In bacteria, genes are arranged singly or in operons
Operon: a group of genes adjacent to each other on the
chromosome that are transcribed from a single promoter into a single
mRNA molecule. Operons are not found in eukaryotes.
The Tryptophan Operon (example)
Negative regulation of the tryptophan
operon
