Download REGULATION OF PROTEIN SYNTHESIS I. Bacteria

REGULATION OF PROTEIN SYNTHESIS
I. Bacteria
Bacterial genome
! Transcription and translation
! Escherichia coli
mRNAs A,B,YZ
! One double helix in a circle
Polysomes A,B,YZ
! 4288 genes
Proteins A,B,Y,Z
rRNA genes
tRNA genes
Structural protein genes A,B,YZ
B. Control of metabolism
Basic principle: control metabolism by controlling
the amount or activity of appropriate enzymes.
1. Control amount of enzyme protein
2. Control activity of existing enzymes
(allostery)
Control of gene expression and protein
synthesis in prokaryotes
The rate of synthesis of each protein is
orchestrated for balanced growth of generalized
cells, and for differentiation of specialized cells.
Examples:
! C a r b on and energy source
! Amino acids
Example: Lactose utilization in E. coli
! Lactose: glucose-galactose (2 sugars
connected by O bridge)
! Utilization requires uptake (carrier,
"permease") and breakdown to separate
the sugars (beta-galactosidase)
Induction
! Synthesis of carrier and beta-galactosidase
are “induced” by lactose (“inducer”)
! Without lactose, there are three molecules
of ß-galactosidase per cell
! With lactose, there are 3000.
Regulating the rate of enzyme synthesis:
Operon Control
1. Genes for associated enzymes are close
together on the DNA and transcribed onto one
mRNA.
! “Operon”: set of genes that are controlled together.
2. The rate of synthesis of enzymes is regulated by
controlling the rate of synthesis of mRNA from the
operon containing their genes
3. A repressor protein prevents mRNA formation at
the operon genes by blocking the action of RNA
polymerase.
! RNA polymerase starts at the
promoter, one for each operon.
! Repressor protein binds to the
operator, near the promoter.
! Repressor is made, at a low
constant rate, from mRNA from
another gene “i” and is always
present.
! When repressor protein binds to the operator, it
blocks RNA polymerase from the promoter.
Induction
4. An inducer (e.g., lactose) binds to and inactivates
the repressor.
Repressor
! Allosteric protein
! Binding sites for both operator and the inducer
! Inducer eliminates the operator-binding site.
! Once the repressor is gone, RNA polymerase
can bind the promoter and transcribe the genes.
Take away (or use up) the lactose
! Repressor recovers its operator-binding shape
(lactose-repressor binding is reversible)
! Binds operator, stops further RNA synthesis
! Existing mRNA breaks down; enzyme synthesis
stops.
Summary
Before lactose
Lactose added
Lactose used up
Example: Tryptophan synthesis in E. coli
! Tryptophan: an essential amino acid
! Synthesis requires a series of five enzymes;
with genes organized into an operon
! When tryptophan is absent, the enzymes are
produced
Repression
! When tryptophan is present, the enzymes are
not needed
! Tryptophan (corepressor) binds to the
repressor
! Repressor binds to the operator and blocks
RNA polymerase
Summary
Tryptophan
present
Tryptophan
needed
Tryptophan
present again
------------------------- removal of corepressor
There is also a type of
regulation (twocomponent) that is
similar to
phosphorylation
regulation of enzymes:
activation of a kinase
(e.g. by osmotic
change) phosphorylates
a gene activator.
Although the induction of protein synthesis with time
looks smooth in cultures, individual cells vary greatly-some have induced mRNA, some don’t; some have a
lot of protein, some don’t. mRNA and protein levels
don’t correlate, because the mRNA degrades much
more rapidly than the protein (so amount of protein
represents the mRNA history, not presence).
Science 329 (30 July 2010) 518, 533
Generalizations
Many groups of genes (operons) and their proteins
are regulated by inducers and co-repressors.
Operon regulation is often similar to allosteric
regulation of enzymes: binding of a compound
changes the shape of the repressor; binding of a
compound changes the shape of the enzyme.
A recent estimate says that there are
approximately 630-700 operons in the E. coli
genome.
Jacques Monod: “What is true for E. coli is true for an
elephant.” Is this right? See next lecture.