Download Operon

CONTROL MECHANISMS
Sections 5.5
Page 255
Outline
1.
2.
3.
4.
5.
Gene regulation
Operons
lac operon
trp operon
Recap
Gene regulation

Housekeeping genes - Always needed;
constantly being transcribed/translated

Most genes are not housekeeping genes

Gene regulation involves turning certain
genes on or off
◦ depends on whether their products are
required

Regulators are the proteins that switch
genes on or off
◦ activators, or
◦ repressors
Recall…
Protein synthesis occurs in two distinct stages:
1.
Transcription, in the
_________
◦Product: ____
2.
Translation, in the
______________
◦Product: __________
Controlling gene expression
Levels of control
1. Transcriptional
2. Posttranscriptional
3. Translational
4. Posttranslational
Levels (cont’d)
Transcriptional gene regulation
 Regulation of which genes are transcribed
 Can involve control of the rate at which
transcription occurs
Posttranscriptional gene regulation
 Modifications made to primary transcript
in the nucleus
Take a break…
Can you make an educated guess about:
Translational control??
 Post-translational??

Levels (cont’d)
Translational gene regulation
 Control of how often and how rapidly
mRNA transcripts are translated into
proteins
Posttranslational gene regulation
 Modifications made to the polypeptide
chain
Transcriptional regulation
Two classic examples:


lac operon – negative regulation
trp operon – positive regulation
What is an operon?
only in
prokaryotic
cells
a cluster of genes
 all genes are under the control of one set of
regulatory sequences

◦ promoter and operator sequences
◦ regulation is dependent on the presence or absence
of effector molecules
Operator sequence

Sequence to which regulator protein binds

Depending on the regulator, binding to the
operator causes either:
◦ activation of transcription
◦ repression of transcription
Effector
Acts on the regulator protein

Can be an inducer
◦ stimulates transcription

Can be a co-repressor
◦ inhibits transcription
THE LAC OPERON
Lac operon
lactose = glucose +
galactose
 disaccharide found in milk
 E. coli: ß-galactosidase
catalyses the cleavage of
the bond in lactose

Not economical to produce
ß-galactosidase when lactose
is not present
Lac operon (cont’d)
Negative control mechanism
prevents production of
ß-galactosidase if lactose is absent
Structure of lac operon
Cluster of genes: lacZ, lacy, lacA
Each codes a different part of the enzyme
ALL are under the control of one promoter

LacI protein – repressor protein. Blocks
transcription of the lac operon by binding to the
operator.

Physical blockage: Covers part of the lac promoter
Mechanism of regulation
http://www.sumanasinc.com/webcontent/
animations/content/lacoperon.html
Mechanism of regulation
Mechanism of regulation
lactose is absent
•LacI binds to lac
promoter.
•No transcription.
lactose is present
 Lactose binds to LacI
protein.
LacI changes
conformation →
unable to bind lac
operator.
 RNA polymerase can
access lac promoter;
transcription occurs.


What is the effector molecule?
_______________

It acts as an inducer – it induces
transcription by deactivating the
repressor protein.
Negative control

Default: Transcription is off, due to
binding of a repressor protein.

Presence of an effector (inducer) removes
the repressor from the operator region.

Transcription can now occur.
TRP OPERON
The trp operon

Tryptophan – Amino acid

Operon: cluster of five genes encoding
enzymes required for trp synthesis

Transcription of operon is repressed when
concentrations of trp in the cell are high
trp operon (cont’d)
Positive control mechanism prevents
production of tryptophan,
if tryptophan levels are high
http://highered.mcgrawhill.com/olc/dl/120080/bio26.swf
Mechanism of regulation
Mechanism of regulation
trp absent
•trp operon is
transcribed (default)
trp present
•trp itself will bind to the
repressor protein.
•conformational shape
occurs
•allows repressor to bind
to the trp operator →
shuts off transcription

What is the effector molecule?
_______________

It acts as a co-repressor. It binds to a
repressor to activate it in order to repress
transcription.
Positive control

Default: Transcription is on

Presence of an effector (co-repressor)
causes binding of the repressor to the
operator.

Transcription does not occur.
trp vs. lac operons
Default state
Positive
control
Effector
Transcription
is on
Allows repressor to
bind
Negative Transcription
control is off
Removes repressor
Positive vs. Negative Control:
Is the repressor added or taken away?
Recap: Vocabulary
Basic terms:

Gene regulation: Describes any process
that alters the rate of gene expression.

Operon: A cluster of closely-related
genes. They are all controlled by one set
of regulatory sequences.
Types of molecules involved:

Regulators: Molecules that carry out
translational gene regulation.
◦ Classified as either activators, or repressors.
◦ Example: LacI repressor; trp repressor

Effectors: Substances to which regulator
proteins respond.
◦ Inducers or Co-repressors
◦ Example: Lactose, Tryptophan
Regulatory DNA sequences:

Promoter: DNA sequence to which RNA
polymerase binds to begin transcription.

Operator: DNA sequence to which a
transcription factor binds in order to alter
transcription. Close to the promoter.
Homework

Pg. 258 #1-6