Regulation of Gene Expression

Prokaryotes conserve energy and resources by making certain proteins only when they are needed. They can rapidly change expression levels as environmental conditions change

Example: lactose catabolism in E. coli

genomic imprinting

In mammals, DNA methylation patterns in sperm differs from that in eggs at about 200 genes. A given gene in this group may be methylated in eggs but unmethylated in sperm. The offspring would inherit one methylated copy and one unmethylated copy. Most imprinted genes are involved with embryonic development

The operon includes an

operator

Two systems for regulating metabolic pathways

Allosteric regulation of enzyme activity (feedback inhibition) Regulation of transcription—slower, but results in greater savings of energy and resources

Uptake and metabolism of lactose involves three proteins

B-galactoside permease-moves lactose into the cell B-galactosidase-hydrolyses lactose B-galactoside transacetylase-transfers acetyl groups to B-galactosides; role is unclear

The structure of a human transcription complex. Consists of 4 kinds of proteins

Basal factors (green)- TATA-binding protein Binds to promoter, essential for transcription but cannot increase or decrease its rate. Coactivators (tan) Link the basal factors with regulatory proteins called activators (red). Activators (red) bind to enhancer sequences on DNA. Repressor(purple)- binds to "silencer" sequence adjacent to enhancer sequence, the activator that would normally have bound that enhancer is no longer able to do so. The activator is unavailable to interact with transcription complex and initiate transcription

Two kinds of chromatin are visible during interphase

Euchromatin—diffuse and light-staining; unmethylated; DNA is available for transcription Heterochromatin—condensed, dark-staining, methylated; contains genes not transcribed

Transcription in prokaryotes can also be regulated by activator proteins that bind to DNA sequences at or near the promoter and promote transcription, activators can regulate both inducible and repressible systems

activators

stimulate transcription(positive regulation)

activators

Gene expression can be regulated at many different points

before or during transcription, before or during translation, after translation

genes that are expressed most of the time at a constant rate are called

constitutive genes

the repressor only binds in the presence of a

corepressor (causes the repressor to change shape and bind to the operator to inhibit transcription)

catalyzes the removal of methyl groups from cytosine

demethylase

If lactose is removed, the mRNA level goes_______ Response of the bacteria to lactose is at the___________

down level of transcription

bind to the core promoter, then RNA polymerase II binds and initiates transcription.

general transcription factors

The repressor protein gene is constitutive (one that is always active), so the repressor is always present. It binds to the operator, but also has an allosteric binding site for the inducer (allolactose). When the inducer binds, the repressor changes shape and can no longer bind to the operator.

inducer info

compounds that stimulate transcription of specific genes are called

inducers( the genes are inducible genes)

operons that are turned off unless needed

inducible operons

a metabolic substrate(inducer) interacts with a regulatory protein (repressor); the repressor cannot bind and transcription proceeds

inducible system(generally controls catabolic pathways)

The lac operon is inducible: the repressor prevents transcription until a β-galactoside predominates. A repressor protein is normally bound to the operator, blocking transcription. In the presence of lactose, the repressor detaches and allows RNA polymerase to initiate transcription.

lac operon

The_______________is very efficient but activity can be reduced when it is not needed—an example of transcriptional regulation

lac operon

upstream of the lac operon, it is a regulatory gene-it encodes a regulatory protein

lac repressor gene

Methylated DNA binds proteins involved in repression of transcription; heavily methylated genes tend to be inactive (silenced). DNA methylation is usually a stable, long-term silencing mechanism. When DNA replicates, maintenance methyltransferase catalyzes formation of 5-methylcytosine in the new DNA strand

methylation(reversible)

A repressible operon is switched _____ when its repressor is bound to its operator

off

a short stretch of DNA near the promoter that controls transcription, repressor proteins bind at the ______________

operator

The three genes for lactose enzymes form an___________ they are adjacent on the chromosome, share a promoter, and are transcribed together

operon

Gene expression begins at the

promoter

operons that are turned on unless not needed

repressible

a metabolic product (co-repressor) binds to a regulatory protein, which then binds to the operator and blocks transcription.

repressible system(generally controls anabolic pathways)

The trp operon for trytohan synthesis is

repressible, When tryptophan is present in adequate quantities, the cell can stop making enzymes for its synthesis. Tryptophan itself functions is the corepressor: it binds to the repressor, causing the repressor to bind to the trp operator to prevent transcription

prevent transcription(negative regulation)

repressors

encode proteins that are not directly involved in gene regulation

structural genes

Regulatory proteins, that control gene activity

transcription factors