Download UNIT 6 lecture part 3regulation

UNIT 6
PART 3
*REGULATION USING
OPERONS*
Hillis Textbook, CH 11
REVIEW:
Signals that Start and Stop
Transcription and Translation
BUT, HOW DO CELLS CONTROL WHICH
GENES ARE EXPRESSED AND WHEN?
First of all, There is a difference between
regulation in a prokaryote and a eukaryote….
OPERONS - PROKARYOTES

Prokaryotes conserve energy by making proteins only when
needed. The most efficient gene regulation is at the level of
transcription.
A gene cluster with a single promoter is an operon.
An operator is a short stretch of DNA near the promoter
that controls transcription of the structural genes.
1. Inducible operon—turned off unless needed
 In inducible systems—a metabolic substrate (inducer) interacts
with a regulatory protein (repressor); the repressor cannot bind
and allows transcription. Usually control CATABOLIC
REACTIONS
2. Repressible operon—turned on unless not needed
 In repressible systems—a metabolic product (co-repressor) binds
to regulatory protein, which then binds to the operator and blocks
transcription. Usually control ANABOLIC REACTIONS.
LAC OPERON - INDUCIBLE


A compound that induces protein synthesis is an inducer.
When the enzymes are induced, metabolism will
take place.
LAC OPERON – INDUCIBLE
E. coli must adapt quickly to supply of food
(lactose is a dissacharide example)
 Uptake and metabolism of lactose involves three
important -galactoside enzymes
-galactoside is a type of glycosidic bond between
monosaccharides… if this is present, LACTOSE is
present.




If E. coli is grown with glucose but no lactose
present, no enzymes for lactose conversion are
produced.
If lactose is predominant and glucose is low, E. coli
synthesizes all three enzymes.
If lactose is removed, synthesis stops.
LAC OPERON – INDUCIBLE
The lac operon is only transcribed when a galactoside predominates in the cell:
• A repressor protein is normally bound to the operator,
which blocks transcription.
• In the presence of a -galactoside, the repressor
detaches and allows RNA polymerase to initiate
transcription.
The key to this regulatory system is the repressor
protein.
NO lactose, repressor will
not allow transcription =
NO enzymes to metabolize
lactose.
WITH lactose available, the
repressor is removed,
transcription will allow
expression of enzymes to
metabolize lactose
TRP OPERON – REPRESSIBLE
A repressible operon is switched off when
its repressor is bound to its operator.
However, the repressor only binds in the
presence of a co-repressor.
 The co-repressor causes the repressor
to change shape in order to bind to the
promoter and inhibit transcription.
 Tryptophan functions as its own corepressor, binding to the repressor of
the trp operon.

NO trp = no repressor and
transcription takes place,
allowing enzymes to be
synthesized for tryptophan.
WITH trp, the repressor
becomes activated and no
transcription takes place.
UNIT 6
PART 3
*REGULATION USING
TRANSCRIPTON
FACTORS*
Hillis Textbook, CH 11
TRANSCRIPTION FACTORS EUKARYOTES
Genes can be regulated at the level of transcription.
Two types of regulatory proteins, called transcription
factors, control whether a gene is active.
These proteins bind to specific DNA sequences near the
promoter:
1. Negative regulation – prevents transcription
2. Positive regulation – stimulates transcription
THIS IS HOW EUKARYOTIC GENES ARE TURNED “ON”
AND “OFF”!
A repressor protein
prevents transcription
An activator
protein binds to
stimulate
transcription
TRANSCRIPTION FACTORS:




Transcription factors act at
eukaryotic promoters.
Each promoter contains a core
promoter sequence where RNA
polymerase binds.
TATA box is a common core
promoter sequence—rich in
A-T base pairs.
Only after general
transcription factors bind to
the core promoter, can RNA
polymerase II bind and initiate
transcription.
REGULATORY PROTEINS:
Besides the promoter, other DNA sequences can bind
regulatory proteins that interact with RNA polymerase and
regulate transcription.
 Some are positive regulators—activators (DNA sequence is
called an enhancer); others are negative—repressors (DNA
sequence is called a silencer).

ALTERNATIVE SPLICING:
Eukaryotic gene expression can be regulated after
the initial gene transcript is made.
 Different mRNAs can be made from the same gene
by alternative splicing (as introns and exons are
spliced out, new proteins are made).
 Mechanism for generating proteins with different
functions, from a single gene.

TRANSLATIONAL REGULATION:
Three ways to regulate
mRNA translation:
• Inhibition of translation
with miRNAs
• Modification of the 5′
cap end of mRNA can
be modified—if cap is
unmodified mRNA is
not translated.
• Repressor proteins can
block translation
directly—
translational
repressors

Posttranslational aspects of
protein synthesis:
 Polypeptide emerges
from the ribosome and
folds into its 3-D shape.
 Its conformation allows
it to interact with other
molecules—it may
contain a signal
sequence indicating
where in the cell it
belongs (nucleus,
mitochondria, etc.)
 In the absence of a
signal, the protein will
remain where it was
produced.
Protein modifications:
Proteolysis—cutting of a long polypeptide chain
into final products, by proteases.
Glycosylation—addition of carbohydrates to form
glycoproteins
Phosphorylation—addition of phosphate groups
catalyzed by protein kinases— charged phosphate
groups change the conformation of the protein
LAC OPERON - INDUCIBLE
LAC OPERON - INDUCIBLE
TRP OPERON - REPRESSIBLE
TRP OPERON - REPRESSIBLE
TRANSCRIPTION FACTORS:
TRANSCRIPTION FACTORS:
TRANSCRIPTION
FACTORS
(TATA BOX):
REGULATORY PROTEINS:
ALTERNATIVE SPLICING:
TRANSLATIONAL REGULATION:
PROTEIN MODIFICATIONS: