Transcript Gene expression of eukaryotic cells

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 molecule
inducer – 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 bgalactosidase increase 1000x
Lac operon - negative regulation
Lac operon - positive 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
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)
Complicated regulation system
• chromatin changes
•
transcription
•
processing RNA
•
transport to cytoplasm
•
degradation of mRNA
•
translation
•
cleavage, chemical modification
•
protein degradation
Stages in gene expression in eukaryotic cell
1. Chromatin changes
• 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
• 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-originspecific 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
Post-translational modifications
Acid/base - act/inact
Hydrolysis – localization, act/inact
Acetylation - act/inact
Phosphorylation - act/inact
Prenylation - localization
Glycosylation - targeting
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.