molecular

1. Gene Regulations/OPERONS
KHZ 2022

2. The control of gene expression
• Each cell contains genetic material: growth &
development.
• Some genes: needed to be expressed all the time
• These genes: for vital biochemical processes- respiration
• Other genes: not expressed all the time
• They are switched on and off at need

3. Genes and Regulatory Elements
• Structural genes: encoding proteins
• Regulatory genes: encoding products that
interact with other sequences and affect the
transcription and translation of these sequences
• Regulatory elements: DNA sequences that are
not transcribed but play a role in regulating other
nucleotide sequences

4. Levels of Gene Regulation
Four points where genes can be
regulated
- through the alteration of DNA
or chromatin structure
-at the level of transcription
- mRNA processing
- regulation of RNA stability
translation control
- posttranslational modification

5. Operons
• An operon is a group of
genes that are
transcribed at the
same time.
• They usually control an
important biochemical
process.
• They are only found in
prokaryotes.
© NobelPrize.org
Jacob, Monod & Lwoff

6. Gene Regulation in Bacterial Cells
• Operon Structure
• Negative and Positive Control: Inducible and
Repressible Operons
• The lac Operon of E. coli
Mutations in lac
• Positive Control and Catabolite Repression
• The trp Operon of E. coli

7. Operon Structure
• Operon: promoter + additional sequences
that control transcription (operator) +
structure genes
• Regulator gene: DNA sequence encoding
products that affect the operon function,
but are not part of the operon

9. Negative and Positive Control; Inducible and
Repressible Operons
• Inducible operons: Transcription is usually off
and needs to be turned on.
• Repressible operons: Transcription is normally
on and needs to be turned off.

10. The lac Operon of Escherichia coli
• A negative inducible operon
• Lactose metabolism
• Regulation of the lac operon
• Inducer: allolactose
– lacI: repressor encoding gene
– lacP: operon promoter
– lacO: operon operator

11. The lac Operon
 The lac operon consists of THREE GENES each
involved in processing the sugar lactose
 One of them is the gene for the enzyme β-
GALACTOSIDASE
 This enzyme hydrolyses lactose into GLUCOSE
& GALACTOSE
© 2007 Paul Billiet ODWS

12. ADAPTING TO THE ENVIRONMENT
• E. coli can use either glucose, which is a
monosaccharide, or lactose, which is a
disaccharide
• However, lactose needs to be hydrolysed
(digested) first
• So the bacterium prefers to use glucose when
possible

13. The lac Operon of Escherichia coli
• Structural genes
• lacZ: encoding β-galactosidases
• lacY: encoding permease
• lacA: encoding transacetylase
• The repression of the lac operon never
completely shuts down transcription.

17. Four situations are possible
1. When glucose is present and lactose is absent the E. coli
does not produce β-galactosidase.
2. When glucose is present and lactose is present the E. coli
does not produce β-galactosidase.
3. When glucose is absent and lactose is absent the E. coli
does not produce β-galactosidase.
4. When glucose is absent and lactose is present the E. coli
does produce β-galactosidase

18. 1. When Lactose Is Absent
• A repressor protein is continuously synthesised. It sits on a
sequence of DNA just in front of the lac operon, the Operator
site
• The repressor protein blocks the Promoter site : RNA
polymerase settles before it starts transcribing
Regulator
gene
lac operon
Operator
site
z y a
DNA
I
O
Repressor
protein
RNA
polymerase
Blocked

19. 2. When lactose is present
• A small amount of a sugar allolactose is formed within the
bacterial cell. This fits onto the repressor protein at another
active site (allosteric site)
• This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the operator
site. RNA polymerase can now reach its promoter site
z y a
DNA
I O

20. 2. When lactose is present
• A small amount of a sugar allolactose is formed within the
bacterial cell. This fits onto the repressor protein at another
active site (allosteric site)
• This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the operator
site. RNA polymerase can now reach its promoter site
Promotor site
z y a
DNA
I O

21. 3. When both glucose and lactose are
present
• This explains how the lac operon is
transcribed only when lactose is present.
• BUT….. this does not explain why the operon
is not transcribed when both glucose and
lactose are present.

22. • When glucose and lactose are present RNA
polymerase can sit on the promoter site but it is
unstable and it keeps falling off
Promotor site
z y a
DNA
I O
Repressor protein
removed
RNA polymerase

23. Summary
Carbohydrates Activator
protein
Repressor
protein
RNA
polymerase
lac Operon
+ GLUCOSE
+ LACTOSE
Not bound
to DNA
Lifted off
operator site
Keeps falling
off promoter
site
No
transcription
+ GLUCOSE
- LACTOSE
Not bound
to DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
- GLUCOSE
- LACTOSE
Bound to
DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
- GLUCOSE
+ LACTOSE
Bound to
DNA
Lifted off
operator site
Sits on the
promoter site
Transcription

24. The trp Operon of Escherichia coli
• A negative repressible operon
• Five structural genes
• trpE, trpD, trpC, trpB, and trpA―five enzymes
together convert chorismate to tryptophan.

26. Gene Regulation in Eukaryotic Cells: Multiple Levels
• Chromatin remodeling
• Chromatin-remodeling complexes: bind
directly to DNA sites and reposition
nucleosomes
• Histone modification
• Addition of methyl groups to the histone
protein tails
• Addition of acetyl groups to histone proteins

28. Gene Regulation in Eukaryotic Cells: Multiple Levels
• Acetylation of histones: flowering in
Arabidopsis
• Flowering locus C (FLC) gene
• Flowering locus D (FLD) gene

33. Protein Arrays