Control of Gene Expression

E. coli lac Operon

lacYDNA

Lac repressor

Regulatory gene

lacI

Binds Lac

repressor

Binds RNA

polymeraseTranscription

initiation site

Promoter Operator

Sequences that control the

expression of the operon

!-Galactosidase

lacZ

lac operon

Permease Transacetylase

Transcription

termination site

lacA

Regulation of Inducible lac Operon

lacADNA

mRNA

Lac

repressor

(active)

lacI

a. Lactose absent from medium

RNA polymerase

cannot bind

to promoter

Promoter Operator

Transcription

blockedWhen lactose is absent from the

medium, the active Lac

repressor binds to the operator

of the lac operon, blocking

transcription.

lac operon

lacZ lacY

Inducible lac Operon

Transcriptionoccurs

b. Lactose present in medium

DNA

Lac

repressor

(active)

mRNA

Binding site

for inducer

Allolactose

(inducer)

RNA polymerase

binds and

transcribes operon

lacI Promoter Operator

When lactose is present in themedium, some of it is convertedto the inducer allolactose.Allolactose binds to the Lacrepressor, inactivating it so that itcannot bind to the operator. Thisallows RNA polymerase to bindto the promoter, andtranscription of the lac operonoccurs. Translation of the mRNAproduces the three lactosemetabolism enzymes.

Lactose

metabolism

enzymes

lacA

mRNATranslation

lacY

lac operon

lacZ

Inactive

repressor

Repressable trp Operon

mRNA

DNA

a. Tryptophan absent from medium

Trp

repressor

(inactive)

mRNA

Regulatory

gene

trpR

RNA polymerase binds

and transcribes operon

Promoter Operator trpE

Transcriptionoccurs

When tryptophan is

absent from the

medium, the Trp

repressor is inactive in

binding to the operator

and transcription

proceeds.

trp operon

trpD trpC trpB trpA

Tryptophan

biosynthesis

enzymes

Translation

Regulation of the repressable trp Operon

trpE trpD trpC trpB trpAtrpR Promoter Operator

trp operon

b. Tryptophan present in medium

Transcription

blocked

DNA

mRNA

Trp

repressor

(inactive)

Tryptophan

(corepressor)

RNA polymerase

cannot bind

to promoter

Trp repressor

(active)Tryptophan-

binding site

When tryptophan is

present in the

medium, the amino acid

binds to, and activates,

the Trp repressor. The

active repressor binds to

the operator and blocks

transcription.

Cytoplasm

NucleusChromatin

Pre-mRNA

DNADetermines which genes

are translated

Transcriptional regulation

• Chromatin remodeling to

make genes accessible for

transcription

• Regulation of transcription

initiation

Mature RNAs

Mature

RNAs

Determines types and

availability of mRNAs to

ribosomes

Posttranscriptional regulation

• Variations in pre-mRNA

processing

• Removal of masking proteins

• Variations in rate of mRNA

breakdown

•RNA interference

Initiation

of protein

synthesis

Ribosome

Determines rate at which

proteins are made

Translational regulation

• Variations in rate of

initiation of protein

synthesisNew poly-

peptide

chains

Finished

proteins

Protein

breakdown

Determines availability of

finished proteins

Posttranslational regulation

• variations in rate of protein

processing

• Removal of masking

segments

• Varieties in rate of protein

breakdown

Chromatin Remodeling

Promoter

Promoter not

accessible to

proteins for

transcription

initiation

Promoter now

accessible to

proteins for

transcription

initiationGene

Promoter

Remodeling

complex

Chromatin remodeling

exposes promoter

Activator

Gene

Regulatory

sequence Nucleosomes

Organization of Eukaryotic Gene

DNA

Regulatory

sequences

Enhancer

Promoter

proximal

elements

(regulatory

sequences)

Promoter

proximal

region

TATA

box

Promoter

5' UTR

Transcription unit of gene

Exon Exon ExonIntron Intron

3' UTR

Transcription Complex on the Promoter

Transcription

begins

Initial general

transcription factor

Additional general

transcription factors

Transcription complex

TATA box

Promoter

RNA polymerase

Site where

transcription starts

DNA

DNA

The first general

transcription factor

recognizes and

binds to the TATA

box of a protein-

coding

gene’s promoter.

1

2 Additional general

transcription factors

and then RNA

polymerase add to the

complex, and then

transcription begins.

Interactions Between Activators

Enhancer

Activators

Coactivator (multiprotein complex)

Interaction between

activators at the enhancer,

coactivator, and proteins

at the promoter and

promoter proximal region

PromoterPromoter

proximal

region

Transcription

initiation site

Gene

Maximal

transcription

Gene

DNA

loop

Activators

Combinatorial Gene Regulation

A unique combination of activators controls gene A.

DNA

Gene A,controlled byactivators 2, 5,7, and 8binding toregulatorysequences inits enhancer

Gene A

TranscriptionActivators 2 5 7 8

2 5 7 8

Enhancer

Enhancerregulatorysequences

Combinatorial Gene Regulation

Gene B

TranscriptionActivators 1 5 8 11

Enhancer

Enhancerregulatorysequences

DNA

1 5 8 11

A different combination of activators controls gene B.

Gene B,

controlled by

activators 1, 5, 8,

and 11 binding

to regulatory

sequences in its

enhancer

Hormonebound toreceptor

Steroidhormone

Steroidhormonereceptor

Protein

DNA

RNA

polymerase

Transcription

Gene controlled

by the steroid

hormone

Steroid hormone

response element

Pre-mRNA

mRNA

mRNA +

ribosomes

DNA methylation silences genes

!The hemoglobin genes for instance are highly methylated

and thus silenced in most vertebrate body cells except red

blood cells.

!DNA methylation sometimes silences large blocks of

genes or even whole chromosomes like one of the X

chromosome in female mammals (Barr bodies)

!DNA Methylation underlies genomic imprinting in which

either the paternal or maternal allele of a particular gene is

silenced.

Histone acetylation activates

genes

The DNA around acetylated histones is less tightly

wrapped around the histones in the nucleosome and thus

more accessible to DNA binding proteins including

transcription factors and RNA polymerase.

Nucleus

Cytoplasm

Other proteinsbind and degradeone RNA strand

Dicer

DicerPrecursor RNAfolds intostem-loop

miRNA gene

miRNAprecursor

miRNA binds tocomplementarytarget mRNA

Degradation

of mRNA

Inhibition

of translation

Stepped Art

Double-strandedRNA

siRNA

Protein Degradation

Development of Colorectal Cancer

Colon

Colon wall

Loss oftumor-suppressorgene APC (orother)

Normal colonepithelial cells

Small benigngrowth (polyp)

Larger benigngrowth (adenoma)

Activation ofras oncogene

Loss oftumor-suppressorgene DCC

Loss oftumor-suppressorgene p53

Additionalmutations

Malignant tumor(carcinoma)