REGULATION OF GENE EXPRESSION

1. REGULATION OF
GENE EXPRESSION

2. DEFINITION
 Gene expression refers to the multistep process that
ultimately results in the production of a functional
gene product, either ribonucleic acid (RNA) or protein
Regulated genes are expressed only under certain
conditions
It is the basis for cellular differentiation,
morphogenesis, and adaptability of any organism

3. LEVEL OF GENE REGULATION
 Prokaryotes: Transcription
 Eukaryotes : Transciption and post transcriptional
and post-translational processes

4. Control of gene expression.

5. REGULATORY SEQUENCES AND
MOLECULES
Regulation of transcription is controlled by regulatory
sequences of DNA
 Embedded in the noncoding regions of the genome.
The interaction between these DNA segments and
regulatory molecules (transcription factors) can
engage or repress the transcriptional machinery

6. TYPES OF TRANSCRIPTION FACTORS
a) Cis acting elements:
 Regulatory sequences flanking a gene
 Influence gene expression only on the same
chromosome
b) A trans-acting factor
 Proteins which transit through the cell from its site
of synthesis to its DNA-binding site

7. Cis-acting elements and trans-acting molecules

8. REGULATION OF PROKARYOTIC GENE
EXPRESSION
 Primarily at the level of transcription
 By the binding of trans-acting proteins to cis-acting
regulatory elements on their single DNA molecule
 Affects the initiation or premature termination of
transcription

9. OPERON
 The structural genes that code for proteins involved in
a particular metabolic pathway are often found
sequentially grouped together along with the cis-
acting regulatory elements
 The genes are, thus, coordinately turned on or off as a
unit.
 This entire package is called as operon
 Eg, Lactose operon, tryptophan operon

10. Role of operators in prokaryotic
transcription
 OPERATOR: A segment of DNA that regulates the
activity of the structural genes of the operon
 REPRESSOR: As long as the repressor is bound to the
operator, no proteins are made.
 INDUCER: binds to the repressor, causing the
repressor to change shape so that it no longer binds
the operator.

11. COMPONENTS OF REGULATION
RNA Poly.
Promoter Operator
Repressor
Gene/
Operon

12. OPERATOR
REPRESSOR
BINDS
OPERATOR
R.P III CANT
PROCEED
HALTS
TRANSCRIPTION
INDUCER
CHANGES
REPRESSOR
SHAPE
REPR.CANT
BIND OPERATOR
TRANSCRIPTION
PROCEEDS

13. LACTOSE OPERON
It contains the genes that code for three proteins
involved in the catabolism of the disaccharide lactose
1. lacZ gene: β-galactosidase
2. lacY gene : permease
3. lacA gene : transacetylase

14. THE REGULATORY COMPONENTS
The regulatory portion has 3 sites:
1. Promoter : Binds RNA polymerase
2. Operator (O) site : Binds repressor encoded by lac-l
gene
3. CAP site: Binds cAMP & CAP (Catabolite Activator
Protein)

15. Regulatory Sites & Their Binding Factors
Lac-l CAP Site Promoter Operator Gene
RepressorRNA Poly.
C AMP +
CAP

16. PRE-REQUISITES FOR GENE EXPRESSION
The lacZ, lacY, and lacA genes are expressed only when:
 The O site is empty
 The CAP site is bound
Lac-l CAP site Promoter Operator Genes
RNA
Poly.
C AMP
+CAP

17. THREE SITUATIONS
1. ONLY GLUCOSE AVAILABLE
2. ONLY LACTOSE AVAILABLE
3. BOTH AVAILABLE

18. I.ONLY GLUCOSE
Glucose  inactive adenyl cyclase no cAMP CAP
remains unbound CAP site remains empty
promoter cannot initiate transcription
OPERON REPRESSED

19. I.ONLY GLUCOSE

20. II.ONLY LACTOSE
1. Absence of glucose, adenylyl cyclase is active cAMP
made  bind CAP protein
 The cAMP–CAP complex binds CAP site
 RNA polymerase initiates transcription
2. Lactose allolactose (Inducer) induces repressor’s
shape change cant bind opertaor operator
empty RNA polymerase can proceed
OPERON INDUCED

21. II.ONLY LACTOSE

22. III.BOTH LACTOSE AND GLUCOSE
ALTHOUGH
Repressor not bound to the operator region
BUT
Glucose  inactive adenyl cyclase no cAMP CAP
remains unbound CAP site remains empty
promoter cannot initiate transcription
OPERON REPRESSED

23. III.BOTH GLUCOSE & LACTOSE

24. Regulation at the level of ribosomes
I.Stringent response in Ecoli:
 Regulation in response to amino acid starvation is
known as the stringent response.
 The binding of an uncharged transfer RNA (tRNA) to the
A site of a ribosome
 Triggers a series of events that leads to the production of
a polyphosphorylated guanosine, ppGpp.
 Catalyzed by stringent factor (RelA)
 Elevated levels of ppGpp result in inhibition of rRNA
synthesis

25. Events after empty tRNA binds A site

26. II.Regulatory ribosomal proteins
 Operons for ribosomal proteins (r-proteins)
 Can be inhibited by an excess of their own protein
products
 For each operon, one specific r-protein functions in
the repression of translation
 It binds Shine-Dalgarno (SD) sequence and inhibits
binding of the small ribosomal subunit to the SD
sequence.
 Also binds to rRNA and with a higher affinity than for
mRNA again inhibiting translation
 If the concentration of rRNA falls, the r-protein then
is available to bind its own mRNA and inhibit its
translation.