Download Gene Expression

การควบคุมการแสดงออกของยีนในเซลล์
โปรคาริโอต
(Control of Gene Expression in
Prokaryotes)
ผศ.ดร. ม ัญชุมาส เพราะสุนทร
Gene Expression

Expression of the genetic information
that the DNA contains
DNA

RNA
protein
High cost of protein synthesis
\Regulation of gene expression is essential to
making optimal use of available energy
Processes that affect the
concentration of a protein

the most efficient
place for regulation
is at the beginning of
the pathway

regulation at the level
of transcription
Principles of gene regulation

constitutive gene expression


genes for the enzymes of central metabolic
pathways
(housekeeping genes)
regulated gene expression

levels of some gene products rise and fall
in response to molecular signals


Induction
repression
inducible enzymes
repressible enzymes
RNA polymerase binds to DNA at
promoters


regulation of transcription
how RNA
polymerase interacts with a promoter
RNA polymerase-promoter interaction
influences the rate of transcription


differences in promoter sequences
regulatory proteins : interfering with the
interaction between RNA polymerase and the
promoter
Transcription initiation is regulated
by proteins that bind to or near
promoters

Types of proteins that regulate
transcription initiation


repressors : impede access of RNA polymerase to
the promoter
activators : enhance the RNA-promoter
interaction
(non-housekeeping genes)
Most prokaryotic genes are
regulated in units called
“operon”


gene cluster, promoter
additional sequences that function
together in regulation
Lactose (lac) operon

lac operon includes the genes for



b-galactosidase (Z)
permease (Y)
transacetylase (A)
Lactose
metabolism
in E.coli
b-galactosidase is an inducible
enzyme
C-source
Glucose/glycerol
Lactose

\ b-galactosidase
b-galactosidase
< 10 molecules/cell
several thousands
molecules/cell
is an inducible enzyme
lac operon in the repressed and
induced states
Inducers of the lac operon


allolactose
IPTG
Inducible catabolic operons are
globally regulated by CAP protein
containing cAMP


if both glucose and lactose are present
“catabolite repression”


prevents expression of the genes for
catabolism of lactose, arabinose and other
sugars in the presence of glucose
the effect of glucose is mediated by


cAMP
CRP (cAMP receptor protein) or
CAP (catabolite gene activator protein)
When glucose is absent, CRP (CAP) binds to
a site near the lac promoter and stimulating
RNA transcription 50 fold
Combined effects of glucose and
lactose on expression of the lac
operon
Control of ribosomal protein
synthesis is at the level of
translation


an increased cellular demand for protein
synthesis
increasing the number of ribosomes
cells must coordinate the synthesis of
ribosomal proteins (r-proteins) and rRNA


the r-protein operons are regulated through a
“translational feedback mechanism”
also functions as a “translational repressor”

at least one protein encoded by each
ribosomal protein operon serves as a
“translational repressor”


binds to mRNA near the initiation site
blocks translation of all the genes


the rate of r-protein synthesis is kept in
balance with rRNA availability
rRNA synthesis responds to


cellular growth rate
availability of crucial nutrients, particularly amino
acids
Regulation coordinated with amino acid
concentrations is called “stringent
response”

when amino acid
concentrations
are low
rRNA synthesis is
halted

ppGpp is
synthesized from
ATP and GDP
during amino acid
starvation