Regulation of gene
expression
Premedical - Biology
Regulation of gene expression
in prokaryotic cell - Operon units, system of
negative feedback
in eukaryotic cell - at any stage of gene
expression and proteosynthesis. Noncoding
RNAs are important.
Operon model
is a functional unit common in bacteria and
phages. Activation and inhibition of
transcription are regulated in response of
conditions in environment.
Prokaryotic genes do not have introns and exons.
Operon
• coordinately regulated clusters of genes,
which are transcribed into one mRNA
(polygenic mRNA, polycystron transcript)
• genes for particular metabolic pathway,
regulated by common promotor and ordered
on DNA following each other
Escherichia coli
Lac operon, Trp operon – model systems =
metabolic pathways of
• utilization of lactose gen lacZ, lacY, lacA, catabolic
pathway with negative and positive regulation
• enzymes for TRP synthesis, anabolic pathway
with negative regulation
each operon consists of
•
•
•
•
promoter (for RNA polymerase)
operator (for repressor)
several structural genes
terminator
repressor = allosteric protein encoded by
regulatory gene
co-repressor = product molecule
inducer = substrate molecule
Tryptophan operon
Lac operon - negative regulation
• regulatory gen produces repressor, which
binds operator and causes that RNAP is not
able to initialize transcription
• in the presence of lactose repressor is
released from operator. The repressor is
changed by inducer / lactose
RNA polymerase starts the transcription. In 2-3 minutes the amount of
enzymes is increased 1000x
Lac operon - negative regulation
Lac operon - positive regulation
• In the presence of glucose, E. coli preferentially
uses glucose for decomposing.
• If is low level of glucosis, the cAMP is
increased.
• CAP „Catabolite activator protein“ in the
presence of cAMP attaches promotor and
activates the transcription.
• CAP is allosteric regulatory protein
Lac operon - positive regulation
Summary:
Lac operon is active
only in time, when the
activator CAP+cAMP is
attached on promotor
(no glucose) and when
is not present represor
on operator (lactose
present)
Gene expression of eukaryotic cells
• each cell maintains specific program /
differential gene expression
• one mRNA carries information for one gene
(monogenic mRNA)
• posttranscription modifications of RNA,
RNA processing and splicing
• regulation system is performed at several
levels = transcription, translation, protein
activation + secretion
more complicated regulative 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, that is why
transcriptional enzymes can not reach the DNA
• Acetylation / deacetylation of histons
• Methylation [cytosin] - inactive DNA is highly
methylated
DNA methylation and histone de-acetylation repress
the transcription.
• 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 regulation
2. Transcription
Transtcribtion factors - proteins that bind DNA and
facilitate or inhibit RNA polymerase to bind. They are
a part of transcription initiation complex.
Transcription factors:
general transcription factors for all protein-coding
genes
specific transcription factors – transcription of
particular genes at appropriate time and place
- enhancers, activators, inhibitors, repressors
Eukaryotic gene and transcript
Cell-type specific transcription:
Genes encoding enzymes of one metabolic
pathway are scattered over the different
chromosomes - coordinated control in
response of chemical signals from outside
the cell. The cell accepts signals by receptors.
Signal transduction pathways activate
transcription activators or repressors.
Signal transduction pathways
3. Processing RNA
Post-transcriptional modifications
Alternative splicing
The same primary transcript, but different mRNA
molecule from it (different exons and introns)
4, 5. transport of mRNA / degradation
Lifespan of mRNA is important for protein synthesis.
6. Translation
At the initiation stage – regulatory proteins bind the
5’ end of mRNA with the cap.
Activation or inactivation of protein factors to initiate
translation
7. Cleavage, chemical modifications
Cleavage
Post-translational modifications
Regulatory proteins [products] are activated or
inactivated by reversible addition of phosphate
groups / phosphorylation
Proteins for cell surface need sugars / 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
Protein for destruction is attached to a small protein
ubiquitin.
Protein complexes
Proteasomes
are places of
degradation.
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.