Regulation of prokaryotic transcription

Question 1

what are allosteric proteins

Answer 1

proteins that have more than one form.

eg. activators bind to DNA in presence of inducer.
eg. repressors dissociate from DNA in presence of inducer.

Question 2

describe negative regulation

Answer 2

involves the action of repressors

• bind to DNA operator site, preventing RNA polymerase froma ttaching to promoter, hence preventing transcription
• relieved by inducer
• may require corepressor in order to bind

Question 3

describe positive regulation

Answer 3

involves action of activator proteins

-when inducer binds, activator can bind to DNA, and enable transcription

Question 4

describe inducible genes

Answer 4

turned off usually, due to presence of a repressor. however, inducer binding can cause repressor dissociation enabling genes to be transcribed.
-catabolic processes, eg. lac operon

Question 5

describe repressible genes

Answer 5

turned on usually,due to presence of activator. however, binding of corepressor dissociates the activator, preventing transcription
-anabolic processes, eg. trp operon

Question 6

describe lac operon basic structure.

Answer 6

controls lactose metabolism in e coli
lac I encodes repressor.
3 structural genes which enable lactose metabolism

Question 7

describe negative regulation of lac operon

Answer 7

lac I encodes repressor which binds to operator site, preventing transcription.
low MW b-galactosides (allolactose) is an inducer, which dissociates the repressor

Question 8

describe positive regulation of lac operon

Answer 8

catabolic activator protein = activator.
the inducer (which enables CAP binding to DNA) is cAMP
CAP-cAMP complex binds to DNA, enablig

Question 9

describe how glucose levels regulate cAMP levels

Answer 9

ATP –adenylate cyclase–> cAMP
high glucose levels inactivate the enzyme, hence reduced cAMP level (therefore, Lac operon is inactive)
low glucose levels activate the enzyme and increase cAMP level

Question 10

describe trp operon

Answer 10

5 enzymes which form tryptophan

Question 11

describe feedback INHIBITION of tryptophan

Answer 11

tryptophan binds to allosteric stie on first enzyme, which inactivates it. as a result tryptophan can’t be made

Question 12

describe feedback REPRESSION of tryptophan

Answer 12

• of initiation: tryptophan acts as a corepressor, binds to repressor which prevents transcription of the tryptophan forming genes
• of termination: mRNA has a leader sequence (contains two tryptophan operons) which makes a leader peptide. if tryptophan is there, only the leader sequence will be made, and RNA polymerase will fall off due to formation of loop structures. if not, the leader sequence wont be transcribed, but the rest of the mRNA will