Prokaryotic Regulation:

Gene RegulationChapter 15

2Gene RegulatioGene RegulationnProkaryotic Regulation:

Bacteria do not need the same enzymes and other proteins all of the time. - They need only: 1. The enzymes required to break

down the nutrients available to them or

2. The enzymes required to synthesize whatever metabolites are absent under the present circumstances.


The Operon Model (Jacob & Monod 1961)

An operon consists of three components:1. Promoter

DNA sequence where RNA polymerase first attaches

Short segment of DNA2. Operator

DNA sequence where active repressor binds

Short segment of DNA


The Operon Model (Jacob & Monod 1961)

3. Structural Genes One to several genes coding for enzymes of a metabolic pathway Translated simultaneously as a block Long segment of DNA

A regulator gene is located outside of the operon. It codes for a repressor that controls whether the operon is active or not.

5Gene RegulatioGene RegulationnRepressible Operons:

The trp Operon - Normally turned ON

If tryptophan (an amino acid) is ABSENT:

Repressor is unable to attach to the operator (expression is normally “on”)

RNA polymerase binds to the promoter

Transcription & translation occur

Enzymes for synthesis of tryptophan are produced

Tryptophan will be produced by E. coli

6The trp Operon

7Gene RegulatioGene RegulationnRepressible Operons:

The trp Operon - Genes repressed

If tryptophan IS present enzymes are not needed and following occurs:

Tryptophan combines with repressor, causing it to change shape, thus acting as a corepressor

Repressor becomes functional

Blocks transcription & synthesis of enzymes and tryptophan is NOT produced

8The trp Operon

9Summary of repressible trp operon

Operon usually ON, must be turned OFF

Repressor Transcriptionbound ? occurs?

NO -------> YES YES -------> NO

*** Corepressors are frequently the products in the pathway. In this case, tryptophan is the corepressor.

10Gene RegulatioGene RegulationnInducible Operons:

The lac Operon - Normally turned OFF

When E. coli is denied glucose & is given lactose instead, it immediately begins to make three enzymes needed for the metabolism of lactose.

These enzymes are encoded by three structural genes which are adjacent to one another on the chromosome. They are controlled by one regulator gene that codes for a one repressor.


The lac Operon - Normal OFF state

If lactose (a sugar that can be used for food) is absent:Repressor attaches to the operatorRNA polymerase cannot bind to promoterTranscription of structural genes is blocked

Enzymes needed to digest lactose NOT made

12The lac Operon


The lac Operon - Induced state

If lactose IS present:

It combines with repressor and renders it unable to bind to operator by causing shape of repressor to change

RNA polymerase binds to the promoter

Transcription of genes occurs

The three enzymes necessary for lactose catabolism are produced

Lactose will be digested by enzymes

14The lac Operon

15Summary of inducible lac operon

Operon usually OFF, must be turned ON

Repressor Transcriptionbound ? occurs?

YES -------> NO NO -------> YES

*** Inducers are frequently the reactants in the pathway. In this case, the lactose is the inducer.

16Gene RegulatioGene Regulationn

The lac Operon - Further control

E. coli preferentially break down glucose. Thus, they have a way to ensure that the lac operon is only turned on maximally when glucose is absent.

This involves use of cyclic AMP which is abundant when glucose is absent.

- Cyclic AMP binds to a molecule called catabolite activator protein (CAP).


The lac Operon - Further control (2)

The cAMP-CAP complex then binds to a CAP binding site next to the lac operon promoter.

• When CAP binds to DNA, the DNA bends.

- This exposes the promoter to RNApolymerase which is now better able to bind to the promoter.


The lac Operon - Further control (2)

When glucose IS present:

There is little cAMP in the cell

- CAP is not activated by cAMP

- lac operon does NOT function maximally and cell will preferentially use glucose as its food source.

19Action of CAP


Animations for the OperonsAnimations for the Operons

http://highered.mcgraw-hill.com/olc/dl/120080/bio27.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio27.swf

Trp Operon

http://highered.mcgraw-hill.com/olc/dl/120080/bio26.swf

lac Operon








Eukaryotic Regulation

A variety of mechanisms to control gene expression:

Five primary levels of control:Nuclear levels

Chromatin Packing

Transcriptional Control

Posttranscriptional Control

Cytoplasmic levels Translational Control

Posttranslational Control

22Regulation of Gene Expression:

Levels of Control in Eukaryotes


Chromatin Structure

Eukaryotic DNA associated with histone proteins Together make up chromatinAs seen in the interphase nucleus

Nucleosomes:DNA wound around balls of eight molecules of histone proteins

Looks like beads on a stringEach bead a nucleosome

The levels of chromatin packing determined by degree of nucleosome coiling

24Levels of Chromatin Structure


Chromatin PackingEuchromatin

Loosely coiled DNAAppears lightly stained in micrographsTranscriptionally active - capable of being transcribed

HeterochromatinTightly packed DNAAppears darkly stained in micrographsTranscriptionally inactive


Chromatin Packing

Barr BodiesFemales have two X chromosomes, but only one is active

Other is tightly packed along its entire length

Inactive X chromosome is called a Barr body

Inactive X chromosome does not produce gene products

27X-Inactivation in Mammalian Females


Transcriptional Control

Transcription controlled by DNA-binding proteins called transcription factors

Bind to a promoter adjacent to a gene

Transcription activators bind to regions of DNA called enhancers. Might be brought near region of promoter by hairpin loops in DNA.

Always present in cell, but most likely have to be activated before they will bind to DNA

29Lampbrush Chromosomes

30Initiation of Transcription


Transcriptional Control (2)

Transposons are specific DNA sequences that have the ability to move within and between chromosomes.

Their movement to a new location sometimes alters neighboring genes by decreasing their expression

- Thus, they can act like regulator genes

- They also can be a source of mutations.



Posttranscriptional control operates within the nucleus on the primary mRNA transcript

Given a specific primary transcript:Excision of introns can varySplicing of exons can varyThus, differing versions of the mRNA transcript might leave the nucleus



Posttranscriptional control may also control speed of mRNA transport from nucleus to cytoplasmWill affect the number of transcripts arriving at rough ER

And therefore the amount of gene product realized per unit time

34Processing of mRNA Transcripts


Translational Control

Translational control determines degree to which mRNA is translated into a protein product

Presence of 5′ cap and the length of poly-A tail on 3′ end can determine whether translation takes place and how long the mRNA is active

- Example: Long life of mRNA in RBCs that code for hemoglobin attributed to presence of 5’ cap and 3’ poly-A tail


Posttranslational Control

Some proteins are not immediately active after synthesis.

Some need to be activated

- Folding and breaking into chains must occur in bovine insulin before it is active

Some are degraded quickly

- Cyclin proteins that control cell cycle


Animations for Eukaryotic ControlAnimations for Eukaryotic Control

http://highered.mcgraw-hill.com/olc/dl/120080/bio28.swfhttp://highered.mcgraw-hill.com/olc/dl/120080/bio28.swf

Control of gene expression in eukaryotes


Transcription Complex and Enhancers



38Gene RegulatioGene RegulationnEffect of Mutations on

Protein Activity

A mutation is a permanent change in the sequence of bases in DNA.

Effects on proteins can range from no effect to complete inactivityGerm-line mutations

Occur in sex cells; can be passed on to future generations

Somatic mutations Occur in body cells; can’t be passed on to future generations

Can lead to development of cancer


Protein Activity

Point Mutations Involve change in a single DNA nucleotide

Changes one codon to a different codonCould change one amino acid for anotherEffects on protein vary:

Drastic - completely nonfunctional Reduced functionality Unaffected


Protein Activity

Frameshift MutationsOne or two nucleotides are either inserted or deleted from DNA

Can lead to completely new codon orderProtein can rendered nonfunctional

Normal : THE CAT ATE THE RAT After deletion: THE ATA TET HER AT

After insertion: THE CCA TAT ETH ERA T

41Point Mutation


Nonfunctional Proteins

Examples of nonfunctional proteins:Hemophilia due to the transposon AluPhenylketonuria (PKU) due to faulty code for one enzyme

Cystic fibrosis due to inheritance of faulty code for a chloride ion channel

Androgen insensitivity due to a faulty receptor for androgens (male sex hormones)


Carcinogenesis

Development of cancer involves a series of mutations:

•Proto-oncogenes – Stimulate cell cycle but are usually turned off. Can mutate and become oncogenes which are turned on all the time.

•Tumor suppressor genes – inhibit cell cycle

Mutation in oncogene and tumor suppressor gene:

Stimulates cell cycle uncontrollably

Leads to tumor formation

44Carcinogenesis


Causes of Mutations

Spontaneous Errors:Happen for no apparent reasonExample of spontaneous germ-line mutation is achondroplasia, a type of dwarfism

Replication Errors: DNA polymerase proofreads new strands Generally corrects errors-1 in 1,000,000,000 replications error occurs


Causes of Mutations

Environmental MutagensA mutagen is an environmental agent that increases the chances of a mutation

Carcinogens - Mutagens that increase the chances of cancer

-Many agricultural & industry chemicals -Many drugs Tobacco smoke chemicals Radiation (X-rays, gamma rays, UV)

47Achondroplasia andXeroderma Pigmentosum

Prokaryotic Regulation:

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