Gene expression in prokaryotes is primarily regulated at the transcription initiation step through the use of operons, which contain clusters of genes controlled by a single promoter. Operons include structural genes, operators that repressors can bind to, and promoters where RNA polymerase binds. Key examples like the lac operon are regulated by repressors that bind to the operator in the absence of an inducer, and activators like CAP that help recruit RNA polymerase to initiate transcription.

1. Regulation of Gene
Expression in Prokaryotes
By / Doaa Elsayed Gad Allah
2014-2015

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3. • It can be defined as the mechanisms that
regulate expression, those mechanisms that
increase or decrease expression of a given gene
as the requirement for its product varies.
• There are various stages at which expression of a
gene can be regulated.
• But the most common stage of regulation
in bacteria (as a prokaryote) is a
transcription initiation step.

4. • Regulation in prokaryotes occurring
through a functioning unit of genomic
DNA known as operon
• which contains a cluster of genes under
the control of a single Promotor.

5. Some definitions
are required
‫؟‬

6. Operon
a unit of coordinated and regulated gene
activity found in prokaryotes, by means of
which the control of the synthesis of a
protein or a group of (usually functionally
associated) proteins is determined.
DNA sequences
operon

7. Operon
DNA sequences
operon
3
structure genesoperatorpromoterregulatory gene
5
It consists of a segment of genomic DNA containing a
structural gene or a linear sequence of structural genes
(which are transcribed as a single unit), promoter &
operator together with one or more regulatory
regions.

8. 3
structure genesoperatorpromoterregulatory gene
5
Promoter
the region at the start of a gene where the
RNA polymerase binds and initiates
transcription.
Many promoters are regulated by “Activators” that
help RNA Polymerase bind DNA and by
“Repressors” that block that binding.

9. 3
structure genesoperatorpromoterregulatory gene
5
Operator
A DNA sequence within the operon to which a
specific repressor protein can bind, thereby
regulating the functioning of a structural gene or
group of genes.

10. Regulatory proteins
Genes are very often controlled by extracellular
signals, in the case of bacteria, this means
molecules present in the growth medium.
These signals are communicated to genes by
regulatory proteins, which come in two types:
- Positive regulators or activators which increase
transcription of the regulated gene
- Negative regulators or repressors which decrease
or eliminate transcription

11. Activation by recruitment of
RNA polymerase

12. • How do activators & repressors
affect the transcription ?

13. • In the absence of regulatory proteins (activators & repressors)
RNA polymerase binds only weakly. This is because one or
more of the promoter elements is absent or imperfect or when
polymerase does occasionally bind, however, it spontaneously
undergoes a transition to the open complex and initiates
transcription.
• This gives a low level of constitutive expression called
• Binding of RNApolymerase is the rate-limiting step in this case.
Basal level of
transcription
the basal level.

14. To control expression from such a promoter, a
repressor needs only bind to a site overlapping the
region bound by polymerase. In that way, the
repressor blocks polymerase binding to the promoter,
thereby preventing transcription.
No transcription

15. • To activate transcription from this promoter, an activator
can just help the polymerase binds the promoter.
• the activator uses one surface to bind to a site on the
DNA near the promoter, with another surface, the
activator simultaneously interacts with RNA
polymerase, bringing the enzyme to the promoter. This
mechanism, often called Recruitment.
Activated level of
transcription

16. EXAMPLES FROM
PROKARYOTES

17. The Lac operon
One of the negative inducible operons
The lac genes of Escherichia coli. (as an example) are
transcribed from a promoter that is regulated by an
activator and a repressor working in the simple way
outlined before.

18. this mRNA is translated to give the three protein products
The three lac genes: lacZ, lacY, and lacA—are arranged
adjacently on the E. coli. genome and are together called
the lac operon. The lac promoter, located at the 5 end of
lacZ, directs transcription of all three genes as a single
mRNA (called a polycistronic message because it
includes more than one gene).

19. The lac Z: gene encodes the enzyme b- galactosidase, which
cleaves the sugar lactose into galactose and glucose.
The lacY: gene encodes the lactose permease, a protein which
transports lactose into the cell.
The lacA: gene encodes thiogalactoside transacetylase, which
rids the cell of toxic thiogalactosides that also get
transported in by lacY.

20. The lac I gene: encodes the Lac repressor.
• Two regulatory proteins are involved: one is a
repressor called the Lac repressor (as previous)
and the other is an activator called CAP
(catabolite activator protein).

21. • Two regulatory proteins are involved: one is a
repressor called the Lac repressor (as previous)
and the other is an activator called CAP
(catabolite activator protein).
• The gene encoding CAP is located elsewhere on the bacterial
chromosome, not linked to the lac genes.

22. • Both CAP and the Lac repressor are DNA-binding
proteins and each binds to a specific site on DNA at or
near the lac promoter
• These genes are expressed at high levels only when
lactose is available, and glucose (the preferred energy
source) is not.

23. In the presence of Glucose
(the preferred energy source)
• Lac repressor can bind DNA and repress
transcription only in the absence of lactose.
• The lac operator overlaps the promoter,
and so the repressor bound to the operator
physically prevents RNA polymerase from
binding to the promoter and thus initiating
transcription to produce lac gene enzymes

24. Glucose
Molecules
35
In the presence of Glucose
(the preferred energy source)
m RNA
Promoter Lac Z Lac y Lac AI gene Operator
RNA
Pol.
Repressor

25. In the presence of Lactose & absence of Glucose
Two events occur:
• Cap binds as a dimer to a site similar in length to
that of the lac operator but different in sequence,
located upstream of the start site of transcription.
When Cap binds to that site, the activator helps
polymerase binds to the promoter by interacting
with the enzyme and recruiting it to the
promoter . This cooperative binding stabilizes
the binding of polymerase to the promoter

26. In the presence of Lactose & absence of Glucose
Two events occur:
• lactose transglycosylation to the lactose
isomer 1,6 allolactose, which acts as an
inducer of the lac operon by binding to
the repressor, changing its conformation
to be unable to bind with the operator

27. 35
m RNA
Repressor
Promoter Lac Z Lac y Lac AI gene Operator
RNA
Pol.
In the presence of Lactose & absence of Glucose
b- galactosidase permease transacetylase
allolactose
Molecules
Polycistronic m RNA

28. Basal level of
transcription
No
transcription
Activated level
of transcription

29. References:
• Molecular Biology of the gene - 6,7 edition
• Oxford dictionary of biochemistry and molecular
biology – revised edition
• Fundamentals of Biochemistry (life at the
molecular level) – 4 edition
• http://www.ncbi.nlm.nih.gov/pubmed/23486479