BI279fs Study Guide - Midterm Guide: Catabolite Repression, Lactose Permease, Adenylyl Cyclase

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6 Nov 2019
Regulation of Gene Expression in E. coli
Gene regulation= control of the amount of product that is made from a specific gene in a
Monod & Jacob found that bacteria can change gene expression based on the
- Operon= a cluster of structural genes involved in a common pathway controlled
by a single regulatory region
Lactose → Glucose
Lactose= inducer, when lactose binds to Lac1 expression turns ON
Default state is ON
Lac1 produces an allosteric repressor protein= negative control
-Lac Z encodes B-galactosidase
-Lac Y encodes lactose permease
-Lac A encodes transacetylase
Genetic Proof of the Operon Model:
Constitutive mutants= structural genes that are always transcribed (→enzymes)
LacI-mutant→ repressor protein can’t bind to operator (trans acting)
LacO(c)→ operator sequence is altered and repressor molecule can’t bind (cis)
Catabolite Repression: Catabolite-activating protein (CAP)- facilitates RNA pol binding
in promoter region = positive control
CAP site is before the Operator
a) No Glucose: CAP binds → transcription ON (more glucose)
b) Glucose: adenyl cyclase is blocked so no cAMP receptor, so CAP can’t bind→ no
extra glucose
five structural genes coding for enzymes involved in tryptophan synthesis
- Repressor gene upstream of operon and independently transcribed
b) No Tryptophan: denatured repressor protein → Transcription ON
c) Tryptophan: repressor binds to tryptophan operator→ Transcription OFF
ARA Operon:
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b) AraC binds to arabinose → araB, araA, araD espression ON
c) No arabinose: AraC binds I and O site → no CAP binding Transcription OFF
Induciblue (LAC, ARA)
•Gene expression is dependent on
environmental conditions
•Inducers allow and repressors block
Repressible (TRP)
•Expression of a gene is blocked when its
product is already present
•Repressor is typically a substrate of the
enzyme encoded by the gene
•Common form of regulation in
biosynthetic (anabolic) pathways
Negative Control
-default is ON
-must be turned off with a repressor
protein (LacI, TrpR)
Positive Control
-default is OFF
-must be turned on with an activator
protein (araC, CAP)
Regulation of Gene Expression in Eukaryotes
-multiple cell types, complex life cycle, more genetic info, DNA w/ histones, 3 RNA pols,
mRNA processing, transcription in nucleus and translation in cytoplasm, mRNA decay
rates can be altered, translational and posttranslational regulation
Chromatin and Histones: transcription only when decondensed (extra regulation)
Specific DNA Sequence (cis-acting)
Binding Protein (trans-acting factor)
General transcription factor (GTF)
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Transcriptional activator (specific)
Transcriptional repressor (specific)
Gal4/UAS System= a gene cluster that codes for proteins/enzymes in the metabolism of
galactose→ glucose
GAL4= tissue specific regulatory sequence UAS= coding sequence for an
isolated protein of interest
****Inducible transcription, positive control*******
- No Galactose:
Gal80p binds to and represses Gal4p OFF
Mig1p gets phosphorylated & sent to the nucleus, then it blocks Gal1 to
suppress expression → catabolite repression OFF
-- Glucose:
Gal3p binds galactose while also binding to and represses Gal80p this
exposes the Gal4p transcription activation domain ON
-discovered in plants by Fire and Mello
TWO types of short RNA molecules:
1) Small interfering RNAs (siRNA)
2) microRNAs
Dicer- cleaves long dsRNA molecules
-short ds RNA associate with RNA-induced silencing complex (RISC) 5’→3’
-RISC targets mRNAs with complementary sequences
Silenced Via:
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