Transcription regulation

Question 1

Why do cells need to regulate transcription?

Answer 1

~ cells require different amounts of different proteins
~ different proteins are produced at different times in the cell cycle
~ some cells only need a few cell-specific proteins

Question 2

Regulatory mechanisms - Prokaryotes

Answer 2

~ promotor strength
~ REPRESSORS/activators/operators/ENHANCERS
e.g. lac/trp operon + adenine riboswitch

Question 3

Regulatory mechanisms - Eukaryotes

Answer 3

~ promoter strength
~ repressors/ACTIVATORS/ operators/enhancers
~ regulatory sequences allow binding of several proteins

Question 4

Repressors

Answer 4

regulatory proteins that decrease transcription

e.g. LexA (SOS genes, bacteria)

Question 5

Activators

Answer 5

regulatory proteins that increase transcription

Question 6

Operators

Answer 6

the regulatory sequences that repressors/activators bind to

Question 7

Enhancers

Answer 7

distal regulatory sequences

~ can be up/downstream of the gene

Question 8

Operons

Answer 8

organise the co-expression of genes that produce proteins which work together
~ produces polycistronic mRNA

Question 9

Polycistronic mRNA

Answer 9

the mRNA of different genes joined together which is translated into the separate proteins encoded by each gene

Question 10

Function of beta-galactosidase

Answer 10

lactose –>galactose + glucose or

lactose –> allolactose

Question 11

Lac operon (lacO)

Answer 11

produces polycistronic mRNA for the following genes:
~ beta-galactosidase (lacZ)
~ permease (lacY)
~ Transacetylase (lacA)
it allows lactose to be respired instead of glucose

Question 12

LacI

Answer 12

~ lac repressor protein
~ binds to lacI and prevents RNA pol. from binding
~ when lactose is present, allolactose binds to LacI, altering its shape and causing derepression
~ negative regulation

Question 13

Lac operon and presence of glucose

Answer 13

glucose inactivates adenylate cyclase = reduced cAMP levels

~ levels of cAMP indicate glucose levels

Question 14

Catabolite activator protein

Answer 14

CAP
~ cAMP receptor protein that binds to DNA in presence of cAMP and bends its structure 90 degrees
~ postive regulation

Question 15

Repressible regulation

Answer 15

operons for biosynthetic pathways are turned off when end product is readily available
e.g. trp operon
~ negative regulation

Question 16

Trp operon (trpO)

Answer 16

contains 5 genes (trpE,D,C,B,A) which encode for 5 enzymes which make tryptophan
~ repressible regulation/negative regulation + attenuation

Question 17

trpR

Answer 17

encodes the aporepressor TrpR

~ tryptophan acts as a corepressor and allows TrpR to bind to operator, preventing transcription of trpO

Question 18

Attenuation

Answer 18

~ reducing expression of the trp operon when levels of tryptophan are high
~ prevents completion of transcription by breaking up the transcripts

Question 19

Attenuation in the trp operon

Answer 19

transcripts terminated within the trpL leader region in mRNA by secondary structure formation –> hairpin formation between regions 1+2, 2+3, 3+4