65 terms

Ch. 11 Gene Regulation Part 1

transcriptional regulation
-process of increasing or decreasing amount of mRNA produced from a gene(s)
cells need to be able to..
recognize & respond to environmental conditions
RNA polymerase
-interacts w/ promoter to begin transcription
-a genetic switch
-sequence specific regulatory protein that bind to operators to initiate transcription
-a genetic switch
-sequence specific regulatory protein that binds to operators to repress transcription
positive regulation
-when activator must bind to its target DNA site in order for transcription to begin
-presence of bound protein is required for transcription
negative regulation
-when a repressor must be prevented from binding to its target site in order for transcription to begin
-absence of bound repressor is required for transcription
allosteric site
-site on regulatory protein that acts as a sensor to set the DNA binding domain as functional or nonfunctional
-interacts w/ allosteric effectors which bind here
allosteric effectors
-small molecules that control ability of activators or repressors to bind to their DNA sites
-binds to allosteric site of a regulatory protein to alter its activity
-ex) lactose changes shape & structure of DNA binding domain of regulatory protein
bacteria prefer to consume...
-glucose over lactose
lac gene expression is required for...
-lactose consumption
metabolism of lactose requires 2 enzymes..
1.) a permease to transport lactose into cell
2.) beta galactosidase to modify lactose into allolactose
-to cleave lactose to yield glucose & galactose
-required for metabolism of lactose
-transport lactose into cell
-encoded by lac Y gene
lac Y gene
-encodes permease
beta galactosidase
-required for metabolism of lactose
-modifies lactose into allolactose
-cleaves lactose to yield glucose & galactose
-encoded by lac Z gene
lac Z gene
-encodes beta galactosidase
-enzyme not required for metabolism of lactose but still in lac operon
-encoded by lac A
lac A gene
-encodes transacetylase
lac I gene
-encodes a lac respressor to block expression of Z, Y, A genes
-not part of lac operon
-in absence of lactose: lac I represses transcription of Lac Z, Y, A
-in presence of lactose: lac I cannot repress transcription of lac Z, Y, A
lac promoter (P)
-site on DNA where RNA polymerase binds to initiate trancription of lac genes Z, Y, A
lac operator (O)
-site on DNA where lac repressor binds to repress transcription of lac Z, Y, A
-located between promoter & Z gene
-genomic DNA containing cluster of genes under control of a single promoter
-set of adjacent genes whose mRNA is synthesized in one piece, PLUS the adjacent regulatory DNA sequences that affect transcription of genes
lac operon & what type of control
-example of negative control of transcription
-5') Promoter, Operator, lac z, lac y, lac a (3'
polycistronic mRNA
-mRNA molecule that encodes for multiple different polypeptides
which lac genes are required for bacteria to metabolize lactose?
-lac Z & lac Y
lac repressor (where does it bind, when & what does it turn off)
-encoded by lac I
-binds tightly only to lac operator prevent transcription of lac Z, Y, A
-lac operon is off when this is bound to operater & is on when this is released
In the absence of lactose....
(when glucose is present)
-lac I represses transcription of lac Z, Y, A
-lac operon switches off
In the presence of lactose.....
(when glucose is absent)
-lac I cannot repress transcription of lac Z, Y, A
-lactose binds to lac I causing repressor to undergo allosteric transition (shape change) causing it to decrease its high affinity for operator
-repressor falls of DNA causing induction of transcription (allowing RNA polymerase to transcribe)
-lac operon switches on
allosteric transition
-when a protein undergoes change in shape after an allosteric effector binds to it
-ex) lactose binds to lac I which causes repressor to undergo this
-relief of repression of a gene under negative control
-ex) when the inducer lactose binds to lac I causing repressor to fall off
-an allosteric effector
-an inducer
-something that interacts w/ repressor to initiate transcription of a gene under negative control
-ex) lactose
discovery of lac system by Jacob & Monod ... -how did lactose increase lac Z activity?
-presence of lactose causes cell to have 1000x lac Z activity so 1000x beta galactosidase was produced
-hypothesis 1: lactose converts an inactive Lac Z precursor into an active form - wrong
-hypothesis 2: new LacZ proteins are produced in response to lactose induction -correct
Jacob & Monod: mutations in structural genes
-affects ability of cell to take up lactose (lac Y) & metabolize it (lac Z)
-Z+ & Y+ are dominant to Z- & Y-
Z+ / Z-
Y+ / Y-
-lac Z expressed: beta gal is produced
-lac Y expressed: permease is produced
Jacob & Monod: mutations in regulatory genes
-affects whether structural genes will be ON or OFF in presence or absence of inducer
-constitutive mutations of Oc & I-
constitutive mutations
-mutations in regulatory genes that causes structural genes to be expressed regardless of presence of inducer
-causes operon to always be on
-ex) Oc & I- mutation
Oc mutation
-constitutive mutation of lac operator
-prevents lac repressor from binding so structural genes are expressed at all times
-cis acting: mutations are restricted to structural genes on same chromosome as the Oc
wild type permease (Y+) is cis to wild type operator (O)
-permease is expressed only when lactose is present
wild type beta galactosidase (Z+) is cis to Oc mutant
-beta gal is constituently expressed regardless of presence of lactose
how does repressor act w/ a wild type: O+ Z+ Y+
-repressor can bind to operator in absence of lactose
can you determine which type of regulation is used in a wild type O+ Z+ Y+ strain?
-NO, from wild type strain, can't differentiate between positive or negative control
Oc Z+Y+, no inducer
-repressor cannot bind to mutated operator
-lac Z, Y, A genes constituently expressed even in absence of lactose
(f') O+ Z- Y+ / (chr) Oc Z+ Y-, with inducer
-F': can't transcribe beta gal
-inducer prevents repressor from binding to wild type operator

-chr: cant transcribe permease

-overall: everything is transcribed
O+ Z- Y+ / Oc Z+ Y-, no inducer
-F': repressor is bound to wild type operator in absence of inducer
-expression blocked

-Chr: no permease produced
-mutant operator prevents repressor from binding to it resulting in constituent expression
I- mutation
-loss of function in I gene causes constitutive expression of lac Z, Y, A
-no repressor
-recessive to I+
-dominant over I-
-trans-acting: can regulate all structural genes on both DNA strands
-generates functional repressor for both strands
I- Z+ Y+ w/o inducer
I- Z+ Y+ w/ inducer
-no repressor: lac Z, Y, A expressed
-no repressor: lac Z, Y, A expressed
(f') I- Z- Y+ / (chr) I+ Z+ Y- with inducer
-only one copy of lac I produces functional repressor
-b/c lactose is present, repressor is inactive & both copies of lac operon are transcribed
(f') I- Z- Y+ / (chr) I+ Z+ Y- w/o inducer
-only one copy of lac I produces functional repressor....
-BUT both genes are repressed b/c lac I+ is trans-acting ... both copies have repressor
-no gene product
super repressor mutations Is
-causes repression even in presence of inducer resulting in structural genes to be constituently inactive
-dominant to I+
-alters allosteric site of lac repressor so it can't bind to lactose
2 environmental conditions that must be satisfied to maximize energy efficiency for bacterial cell
1) lactose must be present in environment
2.) glucose cannot be present in environment
-are mechanisms that prevent cell from synthesizing enzymes for lactose metabolism when both lactose & glucose are present together
catabolite repression
-repression of transcription of lactose-metabolizing genes in presence of glucose is an example of this
-concentration of this controls the lac operon
-breakdown of glucose prevents activation of lac operon
-in high concentration: cells cAMP concentration is low
-as concentration decreases, cAMP increases
-allosteric effector that CAP forms complex w/ to bind & activate lac operon
-high levels of glucose inhibits production of this which prevents activation of lac operon
-high cAMP is signal of low glucose in cell
CAP (catabolite activator protein)
-protein that activates by forming complex w/ cAMP, allowing it to bind to lac promoter
-activates only during low levels of glucose & high levels of cAMP
CAP cAMP complex
-formed when cAMP binds & activates CAP
-binds to lac promoter to recruit RNA polymerase & transcription factors to initiate transcription of lac genes
how does binding of cAMP - CAP complex help binding of RNA polymerase to lac promoter?
-DNA bends when complex is bound which helps binding of RNA polymerase to promoter
breakdown of glucose..
-inhibits high cAMP levels required for formation of CAP - cAMP complex which is needed for lac gene expression
CAP -cAMP system is an example of..
-positive control b/c it activates expression
glucose present (cAMP low), no lactose, no lac mRNA
-lac I binds to operator to repress transcription
glucose present (cAMP low), lactose present
-lac I doesn't bind to operator, no repression
-very little lac mRNA produced
no glucose present (cAMP high), lactose present
-lac I doesn't bind lac operator
-CAP - cAMP complex forms & binds to promoter to activate transcription
-lots of lac mRNA produced
-a repressor decreases transcription
-ex) lac I
-an activator increases transcription
-ex) CAP - cAMP