Week 4 - Signalling & Gene Regulation in
Bacteria
Wednesday, 15 March 2023 20:56
LO1: Understand why signalling and gene regulation is important to cells
LO2: Describe the basic principles of signalling and gene regulation in bacteria
LO3: Differentiate between different signalling and gene regulation systems in
bacteria
- All the bacterial cells in the cover
of this magazine as of the same
species so they're all genetically
identical hence the same size
- They're different colour because of
fluorescent imagining staining for
different genes
- To do that, scientists hooked a
promoter to the gene of interest to
a fluorescent protein
- They made a copy of the promoter in the genome, ensuring the 2 promoters are
identical in terms of radiation, and one promoter they hooked to one
fluorescent protein with a specific colour and the other promoter attached to a
different fluorescent protein of a different colour.
- These different colours tells us that in gene regulation, the two promoters
regulate the expression these 2 genes and similarly, ever cell in there does it to a
different degree
- The intensity at which those 2 promoters fire, despite being exactly identical,
they still behave as individuals
- This is because gene expression intrinsically has a level of noise
○ RNA polymerase has to interact with the promoter
- The LacI repressor has to interact with the regulatory sequences
- Transcription initiation enzymes need to kick in transcription
- SO for all of these events to be coordinated identically, is VERY
difficult so these events are subject to chance fluctuations, leading
to a variability in the number of RNA molecules produced by a given
gene at a given time
- SO as the level of noise increases, the less expression you have of a certain
gene because there's lower copy numbers therefore a smaller change has
appeared
, difficult so these events are subject to chance fluctuations, leading
to a variability in the number of RNA molecules produced by a given
gene at a given time
- SO as the level of noise increases, the less expression you have of a certain
gene because there's lower copy numbers therefore a smaller change has
appeared
- But if you had 10 molecules of mRNA, if you miss one, you're just 10%
down, if you had 1000 molecules of mRNA, if you miss one, you're even
less down
- So at low levels of gene expression, gene expression can be incredibly noisy
resulting in genes being expressed at different levels, even for genetically
identical cells right next to each other, in the same environment will express the
same gene at a different level
- But at higher levels of gene expression, such as when a gene is producing
many molecules of its protein, the noise tends to be reduced due to the
increased robustness of the process
○ Basically at low levels of gene expression, the number of molecules
involved in the process is relatively small hence the 10 mRNA
molecules, 1 being affected = 10% down but 1000 molecules of
mRNA, 1 affected = 0.1% affected, making it more susceptible to
chance events and variability. As the level of gene expression
increases, the number of molecules involved also increases, making
the overall process more robust to random fluctuations
- Cells can do something about this to buffer this issue a little, but they mainly
exploit it since this phenomenon will exist anyway
1. What is the difference between a prokaryote and an eukaryote and why does
this matter in context of gene regulation?
- In a eukaryote there is a nucleus spatially separating the transcription and
translation process, while in prokaryotes there is no separation so mRNA
can be translated right away
- The ribosome that translated the mRNA is already bound as the mRNA is
still being transcribed, enables the cells to stop transcription depending on
where the ribosome sits along the leader sequence
, - Attenuation: the ribosome either reads through the codons the amino acids for
which the biosynthesis operon codes for
- It reads through them, if the tryptophan is present OR it stalls because the
tRNAs aren't charged with the tryptophan and it remains stuck there
- That in turn affects the folding of the 5' region of the nascent mRNA
○ Excess tryptophan = ribosome translated swiftly and covers the area
between sequence 1 and 2 so that sequence 3 and 4 do base pair to
form a hairpin
○ That is a signal for termination of transcription and RNA polymerase
is kicked off so it doesn't transcribe the structural genes
- In contrast, if the ribosome stalls at the tryptophan codons, it stalls at
sequence 1, so sequence 2 base pairs with sequence 3
- Therefore the terminator hair pin is not formed, so transcription continues
through the structural genes
- mRNA then encodes the enzymes necessary for the biosynthesis of
tryptophan
2. Name the components of a signalling pathway. What are their functions?
- Receptor, signal
transducer, and
effector
- Signal can be either
