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Regulation
Regulation
A central theme that runs through many regulatory
mechanisms is the role that low molecular
substances, made by cells or present in
environment, have in mediating control. These low
molecular weight molecules, called effector
molecules or ligands, interact with specific proteins
called allosteric proteins, in a non-covalent fashion,
and alter the properties of these proteins.
Regulation of Gene Expression
new σ
new RNA pol
Use of regulatory proteins
Allosteric protein
Positive regulatory proteins
Activator proteins
Regulatory RNA
Negative regulatory proteins
Repressor proteins
Regulation
5
General Classes of Enzymes
An Inducible Enzyme System
Regulation is specific
Constitutive enzymes
(house keeping genes)
Inducible enzymes
Repressible enzyme
systems
Operon: a collection of genes that are functionally related
and are transcribed as a single unit (polycistronic or
polygenic message), which is under the control of a single
operator/promoter region or regulatory region.
Lactose Operon: Jacob and Monod (J. Mol. Biol 3:318-356, 1961)
Fig. 8.11
Regulation
Lactose metabolism
lactose
(glucose-galactose)
Diauxic Growth
Glucose and lactose
glucose + galactose
β-galactosidase
(β-gal)
Glucose effect
Catabolite repression
lacZ gene
Fig. 8.18
Gratuitous Inducers
β-gal permease (lacY gene)
Transacetylase (lacA gene)
(allolactose)
The lac Operon
Regulation
Regulation of the lac Operon by the
lac Repressor
How explain catabolite repression?
Lactose operon is also under positive control
Activator protein- catabolite activator protein (CAP)
CAP + cAMP -------> CAP/cAMP (active)
Catabolite Repression
Catabolite Activator Protein (CAP)
ATP ---------> c-AMP ----------> AMP
Adenylcyclase
phosphodiesterase
• also called cyclic AMP receptor protein
(CRP)
• brings about the coordinate regulation of
catabolite operons
• exists two forms
– active form when 3’,5’-cyclic adenosine
monophosphate (cAMP) is bound
– inactive form when it is free of cAMP
Fig. 8.19
Regulation
Regulation of the lac Operon by the lac
Repressor and CAP
Phosphotransferase System
Maltose Operon (Regulon)
Sequential Enzyme Induction
Galactose Metabolism
Lactose
(glucose-galactose)
glucose + galactose
Expression controlled
by an activator protein
not a repressor protein
β-galactosidase
Inducer galactose
operon
glucose
Negative and positive control
Fig. 8.15
No CAP/cAMP
involved
Positive Control of Enzyme Induction
Regulation
Fig. 8.17
Tryptophan Operon and Biosynthetic Pathway
Feedback, end-product, inhibition
Repression
Attenuation
Tryptophan Biosynthetic Pathway
Erythrose-4-phosphate
+
7 enzymes
3 enzymes
Chorismic acid
L-tryptophan
PEP
L-phenylalanine
L-tyrosine
Regulation
Enzyme Inhibition by an Allosteric Effector
Feedback (end-product) Inhibition
~1000
Anthranilate synthetase
Chorismic acid
+ or -
Anthranilic
acid
L- trp
More than one effector
Reduce flow of carbon through
trp biosynthetic pathway
Fig. 8.3
Tryptophan Operon
Tryptophan Operon
A repressible enzyme system
Leader region
consists of 162 bp
Trp biosynthetic genes
Trp repressor
gene
Trp
P 0
L
E
D
C
B
A
encodes a 14 aa’s peptide
have ribosome binding site
have a start/stop codon
repressed
Inactive
repressor
+ trp
De-repressed
Active repressor
Operon controlled by attenuation
(lessen in amount)
have regions of homolgy
Regulation
sufficiency
9/10 transcription is
terminated in leader
region
Fig. 8.25a
Tryptophan Operon
Trp limitation
10/10 times transcription
is initiated RNA pol
reads into trp genes
Trp limitation
Anti-terminator loop
Fig. 8.25b
Regulation
Regulation of Branched Pathways
by Feedback Inhibition
Fig. 8.24
Isozymes: catalyze same reaction but are
subject to different effector molecules
Isofunctional Enzyme Inhibition
3-deoxy-Darabinoheptulosonate7-phosphate
Fig. 8.5
Regulation
Need all effectors associated
with enzyme to have inhibition
Assimilation of Inorganic Nitrogen
Concerted feedback inhibition
Each effector
binding inhibits
enzyme by a
certain percent
Two Mechanisms:
1. Glutamic dehydrogenase
α-ketoglutarate + NH3 -----> glutamic acid
2. Second mechanism uses a 2 enzyme system: L-glutamine
synthetase and glutamate synthetase or GOGAT enzyme.
GS
Glutamic acid + NH3 + ATP -------> glutamine
GOGAT
Glutamine + α -ketoglutarate --------> 2 glutamic acids
AMP-adenyl transferase
Glutamine Synthetase
Cumulative Feedback Inhibition
Covalent modification
• Enzyme composed of 12 identical protein subunits.
• Each subunit has 8 distinct allosteric sites (96 allosteric sites).
• As each effector molecule binds get a % inhibition in enzyme
activity.
•Enzyme is also regulated by covalent modification. To have
cumulative feedback inhibition each of the 12 subunits must first be
covalently modified.
less active
More sensitive
to FB inhibition
PII + UTP -------> PII-UMP
low
Glutamine/α-ketoglutarate
Fig. 8.6
Regulation
Flagella Phase Variation
Salmonella typhimurium
Site specific recombination
Riboswitches:
Found in few bacteria, plants,
and fungi
Regulation by RNA’s
sRNA’s: 40-400 nt in lenght