Gene expression in prokaryotes is often controlled through operons, which group functionally related genes together so they can be coregulated. A classic example is the lac operon in E. coli, which contains genes needed for lactose utilization. The lac operon is regulated by a repressor protein that binds to the operator region and blocks transcription when lactose is absent. In the presence of lactose, it binds the inducer and falls off the operator, allowing transcription. Similarly, the trp operon contains genes for tryptophan synthesis and is negatively regulated by a repressor.

1. Gene Expression in Prokaryotes

2. Gene Control in Prokaryotes
 One way in which prokaryotes control gene
expression is to group functionally related genes
together so that they can be regulated together.
 This grouping is called an operon.
 The clustered genes are transcribed together
from one promoter giving a polycistronic
messenger.

3. An operon can be defined as a cluster gene that encode
the proteins necessary to perform coordinated function

4. – Types of operon control
– Inducible operon (lac operon)
– Repressible operon (trp operon)

5. An Lac operon consists of:
– a promoter (binding site for RNA
polymerase)
– a repressor binding site called an operator
that overlaps the promoter.
– structural genes

6.  Lac operon

7. Low lactose
 Negative control in lac operon

8. DNA
Protein
Inactive
repressor
aloLactose
Enzymes for lactose utilization
RNA polymerase
mRNA
High lactose

9. 2- Positive Control of the lac operon
What happens when both glucose and
lactose levels are high?
 Since the inducer is present, the lac operon
will be transcribed.
However the rate of transcription is very slow
(almost repressed) because glucose levels
are high and therefore cAMP levels are low.

12. TRP operon
 The trp operon encodes the genes for the
synthesis of tryptophan.
 As with all operons, the trp operon consists of
the promoter, operator and the structural genes.
 It is also subject to negative control by a
repressor

13.  In this system, unlike the lac operon, the gene for the
repressor is not adjacent to the promoter, but rather is
located in another part of the E. coli genome.