BSC 219
Prokaryotic gene expression
10/25/12
16.2 Many
Aspects of Gene Regulation Are Similar in Bacterial and Eukaryotes
Genes and
Regulatory Elements
Structural
genes: encoding proteins
Regulatory
genes: encoding products that
interact with other sequences and affect the transcription and translation of
these sequences
Regulatory
elements: DNA sequences that are not
transcribed but play a role in regulating other nucleotide sequences
Genes and
Regulatory Elements
Constitutive
expression: continuously expressed under
normal cellular conditions
Positive
control: stimulate gene expression
Negative
control: inhibit gene expression
Level of
Gene Regulation
¥Many levels at which gene expression may be regulated
¥DNA access
¥Transcription
¥RNA modification and stability
¥Translation
¥Post-translational modification
DNA-Binding
Proteins
Domains: 60 ~ 90 amino acids, responsible for binding to DNA, forming
hydrogen bonds with DNA
Distinctive
types of DNA-binding proteins based on the motif
Motif: within the binding domain, a simple structure that fits
into the major groove of the DNA
Helix
turn helix-bacterial, binds major groove
Zinc
fingers-eukaryotic, binds major groove
Leucine zippers-eukaryotic, binds adjacent major grooves
16.3 Operons Control Transcription in Bacterial Cells
Operon: promoter + additional sequences that control transcription
(operator) + structure genes
Regulator
gene: DNA sequence encoding products
that affect the operon function, but are not part of
the operon
Negative
and Positive Control; Inducible and Repressible Operons
Inducible
operons:
Transcription is usually off and needs to be turned on.
Repressible
operons:
Transcription is normally on and needs to be turned off.
Negative
and Positive Control; Inducible and Repressible Operons
Negative
inducible operons:
The control at the operator site is negative. Molecule binding is to the
operator, inhibiting transcription. Such operons are
usually off and need to be turned on, so the transcription is inducible.
Inducer: small molecule that turns on the transcription
Negative
and Positive Control; Inducible and Repressible Operons
Negative
repressible operons: The control at the operator site is negative. But such
transcription is usually on and needs to be turned off, so the transcription is
repressible.
Corepressor: a small molecule that binds to the repressor and makes it
capable of binding to the operator to turn off transcription
Negative
and Positive Control; Inducible and Repressible Operons
Positive
inducible operon
Positive
repressible operon
The lac Operon of E. coli
A
negative inducible operon
Inducer: allolactose
lacI: repressor encoding gene
lacP: operon
promoter
lacO: operon
operator
The lac Operon of E. coli
Structural
genes
lacZ: encoding β-galactosidases
lacY: encoding permease
lacA: encoding transacetylase
The repression of the lac
operon never completely shuts down transcription.
Positive
control and catabolite repression
Catabolite
repression: using glucose when available,
and repressing the metabolite of other sugars
This
is a positive control mechanism: The
positive effect is activated by catabolite activator
protein (CAP). cAMP is binded to CAP, together CAP–cAMP complex binds to a site
slightly upstream from the lac gene promoter.
Positive control
and catabolite repression
cAMP – adenosine-3′,5′-cyclic monophosphate
The
concentration of cAMP is inversely proportional to
the level of available glucose.
The trp Operon of E. coli
A
negative repressible operon
Five
structural genes
trpE, trpD,
trpC, trpB,
and trpA – five enzymes together convert
chorismate to typtophane.
16.4 Some Operons Regulate Transcription Through Attenuation, the
Premature Termination of Transcription
Attenuation: affects the continuation of transcription, not its initiation.
This action terminates the transcription before it reaches the structural
genes.
Attenuation
in the trp Operon of
E. coli
Four
regions of the long 5′ UTR (leader) region of trpE
mRNA
When tryptophan is high, region 1
binds to region 2, which leads to the binding of region 3 and region 4,
terminating transcription prematurely.
Makes
a structure that resembles rho independent loop
Attenuation
in the trp Operon of
E. coli
Four
regions of the long 5′ UTR (leader) region of trpE
mRNA
When tryptophan is low, region 2 binds to region 3,
which prevents the binding of region 3 and region 4, and transcription
continues.
16.5
Antisense RNA Molecules Can Affect the Translation of mRNA
Antisense
RNA:
Complementary to targeted partial sequence of mRNA
16.6 Riboswitches Function as Regulatory Elements in mRNAs
Riboswitch: the regulatory sequence of mRNA molecules where molecules
can bind and affect gene expression by influencing the formation of secondary
structure in the mRNA