Regulation of gene expression

Premedical - biology

Regulation of gene expression

in prokaryotic cell - Operon systems,

negative feedback

in eukaryotic cell – regulated at any stage,

noncoding RNAs

Operon model

a unit of genetic function common in bacteria

and phages

activation and inhibition of transcription in

response of enviroment – changes in the metabolic

status of the cell

genetic information is not divided into introns and

exons

Operon

• coordinately regulated clusters of genes, which

are transcribed into one mRNA (polygennic

mRNA, polycystron transcript)

• genes for particular metabolic pathways are

regulated by common promotore and are

ordered onto DNA followed by each other

Escherichia coli

Lac operon, Trp operon – model systems

• utilisation of lactosis gen lacZ, lacY, lacA, inducible

enzymes / catabolic pathway

negative and positive regulation

• enzymes for TRP synthesis, repressible

enzymes / anabolic pathway

negative regulation / switching off

Operon

• promoter (RNA polymerase is bound)• operator (repressor is bound)• several structural genes• terminator

repressor – regulatory gene, allosteric protein corepressor – product moleculeinducer – substrate molecule

Tryptophan operon

Lac operon - negative regulation

• regulatory gen produces repressor, binds

oprator and causes that RNAP is not able to

inicializate of transcription

• in the presence of lactosis, represor releases

operator, low molecular molecule is called

inducer • RNA polymerase starts the transcription. In 2-3 minutes the amount of b-

galactosidase increase 1000x

Lac operon - negative regulation

• In the presence of glucose, E. coli preferentially

uses glucose

• Allosteric regulatory protein „Catabolite activator

protein“

CAP in the presence of cAMP attaches promotor and

activates

• The activity of gene only in presence of cAMP –

low level of glucosis

Lac operon - positive regulation

Summary: Lac

operon is active

only in time, when

the activator

CAP+cAMP is

attached onto

promotor, but when

is not present

represor onto

operator

Lac operon - positive regulation

Gene expression of eukaryotic cells

• each cell maintain specific program /

differential gene expression

• one mRNA carries information for one gene

(monogennic mRNA)

• posttranscription modification of RNA

(removing introns and connecting exons)

• complicated regulation system, performed at

the several levels (transcription, translation,

protein activation + secretion)

• chromatin changes

• transcription

• processing RNA

• transport to cytoplasm

• degradation of mRNA

• translation

• cleavage, chemical modification

• protein degradation

Complicated regulation system

Stages in gene expression in eukaryotic cell

• Heterochromatin is highly condensed -

transcriptional enzymes can not reach the DNA

• Acetylation / deacetylation of histons

• Methylation [cytosin] - inactive DNA is highly

methylated

DNA methylation and histone deacetylation repress

1. Chromatin changes

• DNA methylation

is esential for long-term inactivation of genes during

cell differentiation

Gene imprinting in mamals

• methylation constantly turns off the maternal or the

paternal allele of a gene in early development

• certain genes are expressed in a parent-of-origin-

specific manner

Epigenetic inheritance

2. Transcription

• proteins that bind to DNA and facilitate of inhibit

binding of RNA polymerase

• transcription initiation complex

• transcription factors – general transcription factors

for all protein-coding genes

- specific transcription factors – transcription of

particular genes at the appropriate time and place

- enhancers, promoters associated with a gene

Eukaryotic gene and transcript

• Cell-type specific transcription

• Genes coding for the enzymes of a metabolic

pathway are scattered over different

chromosomes - coordinated control in

response of chemical signals from outside

the cell - receptors

signal transduction pathways activating of

transcription activators or repressors

Signal transduction pathways

3. Processing RNA

Post-transcriptional modifications

Alternative splicing

The same primary transcript, but different the

mRNA molecule / exons and introns

4, 5. transport of mRNA / degradation

Lifespan of mRNA is important for protein synthesis

Enzymatic shortening

6. Translation

At the initiation stage –

regulatory proteins that bind at the 5’ end of the

mRNA

Activation or inactivation of protein factors to initiate

translation

7. Cleavage, chemical modification

Cleavage

Post-translational modifications

Regulatory proteins [products] are activated

or inactivated by the reversible addition of

phosphate groups / phosphorylation

Sugars on surface of the cell / Glycosylation

• Polypeptide chain may

be cleaved into two or

three pieces

• Preproinsulin

• Proinsulin - disulfide

bridges

• Insulin

• Secretory protein

Acid/base - act/inact

Hydrolysis – localization, act/inact

Acetylation - act/inact

Phosphorylation - act/inact

Prenylation - localization

Glycosylation - targeting

Post-translational modifications

Various steps in the synthesis and assembly of collagen fibrils

8. protein degradation

Lifespan of protein is strictly regulated

Marked protein for destruction is attached by a

small protein ubiquitin

Protein complexes

proteasomes

Thank you for your attention

Campbell, Neil A., Reece, Jane B., Cain Michael L., Jackson, Robert B., Minorsky, Peter V., Biology, Benjamin-Cummings Publishing Company, 1996 –2010.