The regulation of gene expression is critical for all organisms. There are two main types of genes - constitutive genes that are always "on" and non-constitutive or luxury genes that are regulated. Gene regulation involves turning genes on and off through transcriptional and translational control. Key elements that regulate genes include regulatory genes, structural genes, and regulatory elements in DNA that control transcription. Regulation can occur at the level of DNA, transcription, mRNA processing, translation, and post-translation. The lac operon in E. coli is an example of negative inducible gene regulation involving a repressor, operator, and inducer to control lactose metabolism genes.

1. • constitutive genes or house keeping genes.
• non constitutive or luxury genes.
• The control of gene expression or protein
synthesis is called gene regulation or it is the
process of turning genes on and off

2. The Regulation of Gene Expression is
Critical for All Organisms
• Genes and Regulatory Elements
• Levels of Gene Regulation

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. Genes and Regulatory Elements
• Positive control: stimulate gene expression
• Negative control: inhibit gene expression

5. Levels of Gene Regulation
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

6. Why is transcription a particularly important level of gene regulation in both bacteria and
eukaryotes?

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

8. 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.

9. Negative and Positive Control; Inducible
operons
• Negative inducible operons: The control at the
operator site is negative. Molecule binding is to the
operator, inhibiting transcription. Such operons are
usually off and need to be turned on, so the
transcription is inducible.
• Inducer: small molecule that turns on the transcription

10. Negative and Positive Control; Repressible
Operons
• Negative repressible operons: The control at
the operator site is negative. But such
transcription is usually on and needs to be
turned off, so the transcription is repressible.
• Corepressor: a small molecule that binds to
the repressor and makes it capable of binding
to the operator to turn off transcription.

11. 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

12. 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.

15. Positive control of lac operon
• It is an additional regulatory mechanism which
allows the lac operon to sense the presence of
glucose, an alternative and preferred energy
source to lactose.
• If glucose and lactose are both present, cells will
use up the glucose first.
• The presence of glucose in the cell switch off the
lac operon by a mechanism called catabolite
reperssion, which involves a protein called
catabolite activator protein (CAP)

16. • CAP only binds in the presence of
derivative of ATP called cyclic Adenosine
Monophosphate (cAMP)
• CAP binds to a DNA sequence upstream of
the lac promoter and enhances binding of
the RNA polymerase
• Transcription of the operon is enhanced

17. • cAMP levels are influenced by glucose.
• The enzyme adenylate cyclase catalyze the
formation of cAMP and is inhibited by glucose.
• When glucose is available in the cell,
adenylate cyclase is inhibited and cAMP levles
are low
• Under these condition CAP does not bind
upstream of the promoter and the lac operon
is transcribed at very low level.

18. • When glucose is low, adenylate cyclase is not
inhibited, cAMP is higher and CAP binds
increasing the level of transcription.
• If lactose and glucose are present together the
lac operon will only transcribed at a low level.