Prokaryotic Transcription Regulation

Question 1

Repressor

Answer 1

regulates transcription by inhibiting it

binds to the insulator

Question 2

Activator

Answer 2

regulates transcription by facilitating it

binds to the enhancer

Question 3

Co-activator/Co-repressor

Answer 3

bridges for activators acting from a distance

Question 4

Effector

Answer 4

binds activator or repressor to control influence on gene transcription
by binding to activator, can cause dissociation of activator from DNA inhibiting transcription or association of activator to DNA inducing transcription

Question 5

Lac Operon

Answer 5

controlled by both negative and positive regulation
lacZ encodes beta-galactosidase, enzyme that catalyzes the hydrolysis of lactose
lacY encodes lactose permeate, required for transport of lactose into the cell
lacP is the lac operon promoter for the above three genes
O is the operator (where the repressor binds)
lacI encodes the repressor and has its own promoter

Question 6

Lac Repressor

negative control of lac operon

Answer 6

coded by lacI (has its own promoter, constitutively expressed)
when lactose is absent, binds to the operator in the Lac operon, preventing expression of the lac genes
when allolactose is present, binds to the repressor, preventing it from binding to the operator, allowing expression of the genes to occur

Question 7

Cis

Answer 7

something that acts locally on the same DNA molecule

example: lac operon

Question 8

Trans

Answer 8

encodes a protein that acts on a separate piece of DNA
(thus, can be replaced by another functional copy of the gene)
example: lacI

Question 9

Positive Control of Lac Operon

Answer 9

CRP (CAP) activates expression in the absence of glucose
CAP site is located upstream of the promoter
cAMP levels are high in absence of glucose, causing cAMP to bind to CAP, allowing this complex to bind to DNA
CAP bound facilitates RNA Pol attachment, allowing frequent transcription of the lac operon (if lactose is also present)

Question 10

When is lac operon turned off?

Answer 10

–when the repressor is bound–
when there’s high glucose (low cAMP) and absence of lactose
when there’s low glucose (high cAMP) and absence of lactose

Question 11

When is there basal lac operon expression?

Answer 11

–when repressor isn’t bound, but CAP isn’t bound–

when there’s high glucose (low cAMP) and lactose is present (repressor unbound)

Question 12

When is there high lac operon expression?

Answer 12

–when the repressor isn’t bound, and CAP is bound–

glucose is low (high cAMP) and lactose is present (repressor unbound)

Question 13

Positive/Negative Regulation of Gene Expression

Answer 13

negative control: repressor inhibits transcription

positive control: activator facilitates transcription

Question 14

Levels of Gene Expression

Answer 14

Off
Basal (low)
High (activated)

Question 15

Lac Permease

Answer 15

encoded by lacY, required for the transport of lactose into the cell

Question 16

Arabinose operon

Answer 16

regulated by CAP and AraC
AraC acts as a repressor and activator for AraBAD operon, works with CAP-cAMP to stimulate RNA Pol binding
looping prevents activation when unbound, when arabinose binds unloops the DNA
CAP/cAMP still induce binding

Question 17

Trp Operon

Answer 17

creates tryptophan when we don’t have enough of it
trp repressor exists (binds to operator when bound to tryptophan) but doesn’t work well
instead uses attenuation to stop synthesis of the entire gene when there’s lots of tryptophan present

Question 18

Attenuation of Trp Operon

Answer 18

attenuate: to turn something off
four regions before the coding region of trp gene, all of which can bind to each other
under normal conditions, 1&2 bind and 3&4 bind
binding of 3&4 regions causes attenuation, and stops transcription
however, when there are few activated tryptophan tRNAs (low levels of trp) the ribosome slows while translating region 1, allowing regions 2&3 to bind; this prevents attenuation and allows entire gene to be transcribed

Question 19

Operator

Answer 19

repressor binding site