L13 - prokaryotic gene expression 2

Question 1

what are promters acted on by and how many

Answer 1

activators and repressors

= many genes are regulated by mutiple factors

Question 2

why do prokaryotes like bacteria prefer repression as a form of transcriptional regulation

Answer 2

prokaryotic DNA is pretty ‘naked’

= nothing to stop transcription anyway = if not controlled transcription is still likely to happen

= better to repress it

Question 3

what gene/promoter does the activator CAP control

Answer 3

lac-operon

= in prescence of cAMP
= when glucose is low

Question 4

what controls the lac operon

Answer 4

CAP activator protein + LAC repressor

CAP - active in low cAMP + high glucose

LAC - active in low glucose + high allolactose

Question 5

what type of promoter is LacP1 / operonand why

Answer 5

weak promoter

= differs a lot from the consesnus sequnce

in absence of other transcription factors = promoter will be ‘off’

Question 6

what do activators do

Answer 6

turn ‘weak’ promoters into ‘strong’ ones

Question 7

what type of promoter would havce a high Kd

Answer 7

weak promoter

Kd is the conc of ligand required to bind to 50% of enzyme

Question 8

what is K2 in terms of promters

Answer 8

K2 is a meaure of isomerisation

= how easy it is to open a transcripton bubble

Question 9

what do activators do

Answer 9

stabilise binding of RNA polymerase to promoter

= decrease Kd

can also increase K2

Question 10

describe the CAP protein and its binding site

Answer 10

homodimer with 2 binding regions
= each monomer has 1

binding regions spaced by 20 base pair palindromic sequence

binds -60 base pairs downstream from Lac-promoter

cAMP binding site + protein-protein activating region

Question 11

how does CAP affect Kd/transcription of Lac

Answer 11

binds to back end of RNA polymerase via amino acids

holds polymerase on promoter to stabilise it

Question 12

what do the specific protein interactions with RNA polymerase mean activatorn proteins have to be sensitive to

Answer 12

location specific

= activation binding sites must be able to interact with the polymerase on promoter
= on same alignment

due to the turning of DNA helix this means even going down by an extra base can be dysfunctional

Question 13

how do repressors work to prevent transcription of a gene

Answer 13

repressor binding siute overlaps with promoter

= polymerase cannot bind
= doesnt matter whether a promoter is strong or weak if polymerase cannotvreach it

Question 14

why will there be some transcription of a certain gene even in prescmce of repressor

Answer 14

leakiness

repressor is only bound by weak interactions to DNA and can disosociate briefly

= polymerase can quickly bind causing very low levels of transcription of a gene

Question 15

describe the lac repressor

Answer 15

tetramer
= dimer of dimers

one set of dimers binds to lac-operon seuqnce preventing polymerase binding = O1

binds to EITHER O2 or O3
= forms loop in DNA
= increases affinity of repressor to O1 binding site

Question 16

how many operator sequnces aree there for lac operon

Answer 16

3
- O1,O2 and O3

Question 17

how does activation of repressors work

Answer 17

in absence if ligand = repressors are bound

when ligand binds they release DNA

Question 18

what is specificity and apply this to repressors with and without ligands

Answer 18

the ability to bind to specific binding site compared to other sites

a repressor inactivated/bound by ligand is STILL more likely to bind to specific site than anywhere else in DNA

Question 19

allosteric regulation

Answer 19

binding of small molecules affecting stucture/binding

Question 20

descibe the affects of allosteric regulation in CAP and Lac repressor

Answer 20

bound form of CAP = high affinity

= cAMP

bound form of Lac repressor = low affinity

= Allolactose