chapter 12 part 1 Flashcards
constitutive (continuous) transcription
occurs for genes needed continuously to perform tasks (housekeeping genes) & are needed for the life of the cell
constitutive genes
actively expressed all the time
inducible genes
expressed only when their proteins are needed by the cell
regulated transcription (inducible)
needed in genes only needed for responses to changing environmental conditions or during certain phases of growth
-induces control of both initiation & amount of transcription
what is the major means of regulating gene expression in bacteria?
transcriptional regulation
what does transcriptional regulation control?
results from interactions between DNA-binding proteins & regulatory sequences of DNA
negative control of transcription
controls binding of a repressor proteins to a regulatory DNA sequence & preventing transcription
-bind to dna sequences called operators through a DNA-binding domain
-binding blocks transcription
-can be turned on or off by binding other components
positive control of transcription
involves binding of an activator protein to regulatory DNA & initiating gene transcription
-also have a dna-binding domain
-binding to dna causes rna polymerase to initiate transcription
-turned on or off
how many domains to acitvator/repressor proteins (TFs) have & what are they?
at least two
-dna binding domain & allosteric domain
what does the dna binding domain do?
it locates & binds operator dna sequence or other target dna sequences
what does the allosteric domain bind?
a molecule or protein, which causes a change in conformation of the dna-binding domain, altering its function
-turns dna binding on or off
-this is properly called allostery
what are the four categories of gene regulation
negative inducible, negative repressible, positive inducible, positive repressible
inducible
the effector molecule causes transcription to START
repressible
the effector molecule causes transcription to STOP
repressor
negative control
activator
positive control
all four catrgories of gene regulation require what?
allosteric effector/inducer/inhibitor/co-repressor molecule
negative inducible
-TF is a repressor
-binding of the effector molecule to the TF causes transcription to START
-effector molecule is an inducer
-lac operon in ex
negative repressible
-TF is a repressor
-binding of the effector molecule to the TF causes transcription to STOP
-effector molecule is a co-repressor
-trp operon is an ex
positive inducible
-TF is an activator
-binding of the effector molecule to the TF causes transcription to START (co-activator)
-effector molecule is an inducer
positive repressible
-TF is an activator
-binding of the effector molecule to the TF causes transcription to STOP
-effector molecule is an inhibitor
what is the most common structural feature of homodimers in bateria?
the helix-turn-helix (HTH) motif, in which two alpha-helical regions interact with the DNA sequences
what are the recognition & stabilizing helix separated by?
a turn in each polypeptide
operons
-Genes in an operon nearly always participate in the same metabolic or biosynthetic pathway
clusters of genes undergoing coordinated transcriptional regulation by a shared regulatory region
where are operons commonly found?
in bacterial genomes
can operons be activated or repressed to control activity of a set of genes?
yes!
-switching mechanisms are typically responsive to environmental cues
what operon of e. coli is responsible for producing three proteins needed for use of lactose
lactose (lac) operon
what is the principle energy source for e.coli & most organisms?
glucose
-this monosaccharide is metabolized directly by the glycolysis pathway
_________ is one of the many sugars that can serve as an alternate carbon source; its utilization is controlled by the lac operon, which is an inducible operon system
lactose
-inducible systems are turned on only when an inducer compound (lactose in this case) is available
what is lactose made of?
glucose & galactose
what bacteria phenotype can grow on media containing lactose as the only sugar?
lac +
-they do this by producing a gated channel (permease) at the cell membrane that allows lactose to enter the cell & the enzyme beta-galactosidase to break the beta-galactose linkage
what does the breakdown of lactose also produce?
allolactose which acts as an inducer compound
what does the lac operon consist of
multipart regulatory region & three structural genes
how many binding sites does the regulatory region have?
-promoter (P) that binds to rna polymerase
-operator (O) (LacO) that binds the lac repressor protein
-the CAP binding site
where are the regulatory regions located
immediately upstream of the start of transcription of the structural genes
lac operon structural genes
-lacZ-encodes beta-galactosidase
(cleaves lactose into glucose & galactose)
-lacY- encodes a permease
(helps transport lactose into the cell
-LacA- encodes transacetylase
(dont know function)
how are the lac operon structural genes transcribes?
as a single, polycistronic mRNA, which is translated to produce the three distinct polypeptides
where is the lacl gene?
next to, but not part of, the lac operon
lacl gene
constitutively expressed (made all the time)
what does lacl encode?
lac repressor protein
lac repressor protein
-is a homo-tetramer
-has a dna-binding domain that binds the the lacO sequence (operator)
-has an allosteric domain that binds the inducer- allolactose
how does the lactose operator bind to the operator?
reversibly
-when allolactose is around, it can come off
what does permease do
allows lactose into the cell, if permease gene is always off, we don’t let lactose in
how would we make allolactose if we dont have a permease?
we never would make allolactose
since there is a very low basal level of transcription from the operon, what does it make this system?
repressor stays on & every once in a while, it comes off randomly & the rna polymerase gets by… makes a low level basal transcription rate always
negative inducible control
when lactose is + & glucose is -, what do we need to use as an energy source?
lactose
if lactose is around, we make allolactose which binds to the allosteric domain of the lac repressor
what does the formation of the inducer-repressor complex alter?
the dna-binding domain of the repressor & prevents it binding the operator
-negative control (repression) is removed
-transcription of lac operon is induced
