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Lecture 3

BIOLOGY 1A03 Lecture Notes - Lecture 3: Permease, Operon, Repressor


Department
Biology
Course Code
BIOLOGY 1A03
Professor
Rosa Da Silva
Lecture
3

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Theme 3 Module 2: Prokaryotic Transcriptional Regulation
Unit 1: Responses to the Environment
-E.coli cells in an environment with glucose and lactose will metabolize all the
glucose before switching to lactose through a highly regulated process
-Metabolic shift in nutrient source is tightly regulated such that they are able
to quickly upregulate the expression of genes that produce lactose-
metabolizing enzymes
-Control of enzyme production is regulated at the transcriptional level
-Transcription of gene sin the E.coli genome are turned-on to accommodate
for this change in nutrient source
-Two important cues detected by E.coli, changes in the glucose levels and
presence of lactose
Transcriptional Responses to the Environment
-Concomitant increase in the amount of
detectable beta-galactosidase and lactose
permease proteins
-Proteins not detectable when E.coli cells are
utilizing glucose, but are soon expressed as
they are required for the transport and
metabolism of lactose
-Two gene products are not expressed until
glucose if fully depleted from the growth
medium – is glucose inhibiting or
repressing the expression of these
genes?
-Once bacterial cells are starting to use
lactose, there is an increase in the expression of the B-galactosidase and
lactose permease proteins – could lactose induce their expression once
glucose is depleted?
Cues Lead to Gene Expression Products
-Proteins produced in response to
environmental cues, and allows for the
e&ective digestion of lactose
- Lactose permease is a transport
protein that sits in the bacterial cell
membrane, allowing for the lactose to
enter the bacterial cells
- B-galactosidase is the cytoplasmically
situated bacterial enzyme that cleaves
imported lactose into glucose and galactose
-Groups of related genes with similar functions are often found clustered
together into operons
-Di&erent from eukaryotes where each gene has its own promoter and
enhancers
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-Organizational di&erence in prokaryotes leads to the ability to control the
transcription of the whole gene cluster as one unit
Bacterial Genes are clustered into Operons
- Jacob & Monod, discovered the
operon
model
for this type of control of gene
expression
-Functionally related genes are organized
into transcriptional units along the
bacterial chromosome
-Gene clusters controlled by a single “on-
o& switch”, that controls the
transcription of the clustered genes
-Coordination is mediated by an operon,
consisting of a promoter, an operator
(on-o& switch) and the coordinated gene
cluster whose products will function in a
common pathway or cellular response
-An operator is a sequence of nucleotides near the start of the operon,
regulated to allow or inhibit transcription
-When the operator is not bound to any transcriptional inhibitor, RNA
polymerase can attach to the promoter
-Transcription in bacteria can give rise to one long mRNA (polycistronic
mRNA) that codes for multiple proteins
The Lac Operon-a Model for
Transcriptional Regulation
- Lac operon is a model for
transcriptional regulation in
prokaryotes
- Regulatory sequences of
transcription within the lac operon
oPromoter that binds the RNA
polymerase complex and the
operator (lacO), the binding site
for a repressor protein expressed
by the lac l coding sequence
-Two main structural genes code for
primary proteins needed to facilitate
lactose metabolism
oLacY gene: lactose permease
oLacZ gene: B-galactosidase
- Lac l gene controls the expression of the lacZ and Y genes
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