chapter 12 part 1 Flashcards
constitutive transcription
occurs for genes needed continuously to perform housekeeping tasks, needed for life of the cell
regulated transcription/inducible genes
for genes only needed for responses to changing environmental conditions or during certain phases of growth
what does regulated transcription include the control of
imitation and amount of transcription
what is the major means of regulating gene expression bacteria
transcriptional regulation
what mechanisms are also important for gene expression
post-transcriptional mechanisms
negative control of transcription
involves binding of a repressor protein to a regulatory DNA sequence and preventing transcription
where do repressors bind
operators through DNA-binding domain
does negative control turn on or off genes
both
positive control of transcription
involves binding of an activator protein to regulatory DNA and initiating gene transcription
binding of activator to DNA causes what?
RNA polymerase to initiate transcription
does positive control turn on or off genes
both
what 2 domains do activator/repressor proteins have?
- DNA binding domain
- allosteric domain
activator and repressor proteins are examples of what?
transcription factors
DNA-binding domain
locates and binds operator DNA sequence or other target DNA sequences
allosteric domain
binds a molecule or protein, which causes a change in conformation of the DNA-binding domain, altering its function
allostery
to turn DNA binding function on or off
4 categories of gene regulation
- negative inducible
- negative repressible
- positive inducible
- positive repressible
inducible
the effector molecule causes transcription to start
repressible
the effect molecule causes transcription to stop
all 4 categories of gene regulation require the participation of an additional…
allosteric effector/inducer/inhibitor/corepressor moelcule
negative inducible
- TF = repressor
- binding of effector molecule to TF causes transcription to start
- effector molecule is inducer
ex. of negative inducible
Lac operon
negative repressible
- TF = repressor
- binding of effector molecule to TF causes transcription to stop
- effector molecule is co-repressor
ex. of negative repressible
Trp operon
positive inducible
- TF = activator
- binding of effector molecule to TF causes transcription to start (co-activator)
- effector molecule is inducer
positive repressible
- TF = activator
- binding of effector molecule to TF causes transcription to stop
- effector molecule is inhibitor
bacterial regulatory DNA sequences frequently have what?
inverted or direct repeats
each polypeptide of a regulatory homodimer interacts with what?
one repeat
most common structural feature of homodimer (repeat) proteins in bacteria
helix-turn-helix motif (HTH)
helix-turn-helix (HTH) motif
two a-helical regions interact with the DNA sequences
a recognition and a stabilizing helix are separated by a ________ in each polypeptide
turn
operon
clusters of genes undergoing coordinated transcriptional regulation by a shared regulatory region
where are operons commonly found?
bacterial genomes
genes in an operon nearly always participate in what?
the same metabolic or biosynthetic pathway
can an operon be activated or repressed
both
switching between activated/repressed are typically responsive to what?
environmental cues
how many proteins does the lactose (lac) operon of E. coli produce
3 - all needed for use of lactose
what is the main energy source for E. coli and most organisms
glucose
what is glucose metabolized by
glycolysis pathway
lactose
one of many sugars that can serve as an alternate carbon source
- utilization controlled by lac operon
lactose is a disaccharide of __________ and __________
glucose and galactose
in lactose, what are glucose and galactose joined by
covalent B-galactoside linkage
bacteria with a lac+ phenotype can grow on media containing what?
lactose as the only sugar
can bacteria with the lac- phenotype utilize lactose?
no
how can lac+ bacteria grow on lactose
produce gated channel (permease) at cell membrane that allows lactose to enter the cell and the enzyme B-galactosidase to break the B-galactoside linkage
glucose produced by lactose breakdown enters _________
glycolysis
galactose is processed to produce _________
glucose
the breakdown of lactose produces a small amount of _________
allolactose
what does allolactose act as
inducer compound
3 binding sites of lac regulatory region
- promoter - binds RNA polymerase
- operator - binds lac repressor protein
- CAP binding site
3 operon structural genes of lac
- lac Z
- lac Y
- lac A
what does lacZ encode
B-galactosidase
- cleaves lactose into glucose and galactose
what does lacY encode
permease
- helps transport lactose into the cell
what does lacA encode
transacetylase
- precise function is mystery
how are lac Z, Y, and A transcribed
as single, polycistronic mRNA, which is translated to produce the 3 distinct polypeptides
what gene is next to the lac operon?
lacI
is lacI part of the lac operon?
no
is lacI constitutively or inducible expressed
constitutively
what does lacI encode
the lac repressor protein
lac repressor protein
- homo-tetramer
- DNA-binding domain that binds to the lacO sequence (operator)
- has allosteric domain that binds to the inducer (allolactose)
what is present even when lactose is absent?
a very low basal level of transcriptional from the operon
how is there always a small amount of transcription for the lac operon
small amount of permeate and B-galactosidase proteins allow for the import of lactose into a cell and production of allolactose
what happens when there are no or low levels of B-galactosidase
no allolactose in the cell, lac repressor protein binds to lacO and prevents transcription
