1.8 - Bacterial Transcription + Lac Operon

Question 1

sigma (σ) factor

Answer 1

polymerase subunit that recognises and binds to promoter sequences on DNA

Question 2

result of sigma factor and RNA polymerase combining

Answer 2

holoenzyme

Question 3

what does binding of sigma allow RNA polymerase to do?

Answer 3

recognise transcription site and strongly bind to DNA

Question 4

what do multiple sigma factors allow for?

Answer 4

different groups of genes to be regulated

Question 5

most abundant sigma factor

Answer 5

sigma 70 (σ70)

Question 6

role of sigma 70 (σ70)

Answer 6

responsible for transcription of housekeeping genes

Question 7

regulator sites

Answer 7

located around promoter, can inhibit or enhance transcription

Question 8

operator site

Answer 8

sequence recognised by repressor genes

Question 9

effect of repressor genes binding to operator site

Answer 9

physically blocks RNA polymerase from binding

Question 10

activator sequences

Answer 10

bind proteins that can help to increase efficiency of RNA polymerase to bind to DNA

Question 11

role of sigma factor within holoenzyme

Answer 11

initiates “melting” of DNA helix (separation of strands to prepare for transcription)

Question 12

what determines whether top/bottom DNA strand is transcribed?

Answer 12

which strand the specific promoter sequence that sigma factor has bound to is on

Question 13

when can transcription of the RNA begin?

Answer 13

when DNA is melted and strands exposed

Question 14

what replaces thymin in RNA transcript?

Answer 14

uracil

Question 15

when does sigma factor disassociate?

Answer 15

as RNA transcript is being created (no longer needed) - free to bind to other RNA polymerase to initiate transcription elsewhere

Question 16

elongation

Answer 16

RNA polymerase continues to add nucleotides to end of RNA transcript until signalled to stop transcribing at termination sequence

Question 17

terminator sequence (2)

Answer 17

1. complementary sequences of this creates hairpin structure, causes RNA polymerase to stall
2. combined with stretch of uracils on end of transcript, there is reduced interaction between DNA, RNA and RNA polymerase
   (causes termination of transcription and release of RNA transcript)

Question 18

effect of transcription occurring in cytoplasm (bacteria)

Answer 18

most of the time, new transcripts exposed to proteins in cytoplasm, can include ribosomes which carry out translation

Question 19

result of bacterial transcripts being exposed to ribosomes in cytoplasm

Answer 19

transcription and translation can occur simultaneously, means RNA transcript generally not modified

Question 20

why do bacteria require tight transcriptional control?

Answer 20

single cell organisms, ensures efficient use of energy and resources

Question 21

operons

Answer 21

polycistronic genes allowing for coordinated expression of multiple genes at same time

Question 22

what are operons under control of?

Answer 22

the same promoter

Question 23

Lac operon

Answer 23

multiple genes that form Lac Operon all involved in metabolism of lactose into glucose and galactose

Question 24

genes within Lac Operon (3)

Answer 24

1. lacZ gene - β-galactosidase
2. lacY gene -galactoside permease
3. lacA gene - β-galactosidase transacetylase

Question 25

default state of Lac Operon

Answer 25

"turned off" through Lac repressor coded by lacI gene (lower case lac, upper case I), occurs when cell has the glucose it needs

Question 26

how is the lacI gene a trans-regulator of the Lac Operon?

Answer 26

has its own promoter and terminator sequence

Question 27

state of Lac repressor (lacI)

Answer 27

always transcribed and expressed

Question 28

role of lac repressor

Answer 28

bonds to operator in Lac Operon preventing RNA polymerase from binding

Question 29

allosteric regulation

Answer 29

regulatory mechanism whereby proteins interacting with sequences often require additional molecules to allow them to bind to DNA

Question 30

how do additional molecules allow DNA-binding proteins to bind (allostery)

Answer 30

bind to DNA-binding protein causing conformational change altering how it binds to DNA

Question 31

what does allostery allow for?

Answer 31

activation or inhibition of gene expression in response to stimuli

Question 32

different effects of allosteric regulation (dependant on DNA-binding proteins) (4)

Answer 32

1. inducible repressors 2. repressible repressors 3. repressible activators 4. inducible activators

Question 33

inducing repressors (allostery)

Answer 33

do not inhibit transcription when bound to effector molecule

Question 34

repressible repressors (allostery)

Answer 34

inhibit transcription when bound to effector molecule

Question 35

repressible activators (allostery)

Answer 35

do not activate transcription when bound to effector molecule

Question 36

inducible activators (allostery)

Answer 36

causes transcription when bound to effector molecule

Question 37

binding of CAP to Lac Operon (3)

Answer 37

1. when cell runs out of glucose, cell produces cyclic AMP from breaking down ATP 2. cyclic AMP can bind to protein cAMP-bound catabolite activator protein (CAP) 3. cyclic AMP acts as inducible activator of CAP, can then bind to CAP binding site upstream of Lac Operon promoter

Question 38

activity of CAP if there's no lactose for the cell to convert into glucose

Answer 38

repressor remains bound to operator to prevent CAP from helping to bind RNA polymerase (no point wasting further energy making enzymes when no lactose to break down)

Question 39

role of allolactose (2)

Answer 39

1. if lactose present in cell it will be metabolised into allolactose by β-galactosidase (always small amount in cell) 2. allolactose acts as inducible repressor, binds to Lac repressor protein, causing conformation change and can no longer bind to operator site (detector system for presence of lactose)

Question 40

result of removal of Lac repressor

Answer 40

RNA polymerase with sigma factor able to bind to DNA, with help of CAP protein. transcription and subsequently translation occur producing protein, this allows lactose to be transported into cell and broken down into glucose

Question 41

activity of CAP and lac repressor when returning to baseline (glucose present) (4)

Answer 41

1. cyclic AMP reduces as glucose becomes main energy/carbon source, meaning less cAMP to bind to CAP 2. means less CAP able to bind to DNA 3. ensures cell not continuously making more enzyme where glucose/lactose present (already converted as needed) 4. eventually lactose/allolactose levels drop and cycle restarts with binding of Lac repressor to operator

Question 42

summarise how Lac Operon regulates expression of genes involved in metabolising lactose to glucose

Answer 42

by using repressor and activator, ensures process is down to efficient manner when required without wasting energy making gene products that won't be used

Question 43

tryptophan operon

Answer 43

operon where there are five genes within it relating to tryptophan biosynthesis

Question 44

what regulates binding of active repressor to operator site within promoter region in tryptophan operon

Answer 44

tryptophan levels (acts as repressible repressor turning off operon)

Question 45

maltose operon action at high maltose levels

Answer 45

maltose binds to maltose activator protein, allowing for expression of genes for maltose metabolism

Question 46

maltose operon action at low maltose levels

Answer 46

less to bind to activator protein, transcription and gene expression of operon is reduced