Ch. 20 Regulation of Gene Expression in Bacteria

Question 1

What does the lac operon encode?

Answer 1

Three proteins that are involved in lactose catabolism (lactose → glucose).

Question 2

What kind of mRNA is produced from the lac operon?

Answer 2

A polycistronic mRNA is produced using one promoter.

Question 3

What does expression of the lac operon depend on? (2)

Answer 3

The availability of lactose and glucose. Only needs to be on when lactose is present and glucose is absent.

Question 4

What is lac expression regulated by? (2)

Answer 4

Regulated by using sequences near the promoter… lacO and CRP-binding site.

Question 5

What is negative control of lac facilitated by?

Answer 5

LacI

Question 6

What is LacI?

Answer 6

A repressor protein that is encoded by lacI (not part of the operon).

Question 7

Where does LacI bind and what does this do?

Answer 7

Binds to lacO sites to prevent transcription of lac (either lacO1, lacO2, or lacO3).

Question 8

What does binding of two sites by LacI do?

Answer 8

Creates a loop.

Question 9

What does allolactose do to LacI?

Answer 9

Allolactose binding to LacI causes conformational change that inactivates DNA binding ability.

Question 10

What is positive control of lac facilitated by?

Answer 10

cAMP receptor protein (CRP); also known as catabolite activator protein (CAP)

Question 11

When is the expression of lac repressed?

Answer 11

Expression of lac is repressed when glucose is available.

Question 12

What does CRP bind to and what does this do?

Answer 12

Binds to the CRP-binding site when cAMP is present and recruits RNApol.

Question 13

How is [cAMP] related to [glucose]?

Answer 13

Inversely proportional

Question 14

What is the state of the operon when lactose is absent?

Answer 14

When lactose is absent, LacI is bound to operators and represses expression. [Glucose] doesn’t matter.

Question 15

What is the state of the operon if glucose is absent?

Answer 15

When glucose is absent, lac is only expressed at a high rate if lactose is present.

Question 16

What else does LacI regulate? CRP?

Answer 16

LacI ONLY regulates the lac operon. CRP is used to regulate many sugar metabolism genes, however.

Question 17

What does CRP serve as?

Answer 17

An general indicator of [glucose] through its association with cAMP

Question 18

How does CRP interact with other genes/operons?

Answer 18

Many genes/operons in bacterial genomes have CRP-binding sites to regulate expression in response to [glucose].

Question 19

Is CRP an activator or a repressor?

Answer 19

It can be both depending on the gene/operon involved.

Question 20

What is the repressor for ara? gal?

Answer 20

Both use unique repressors that bind to operator sites. ara uses AraC and gal uses GalR.

Question 21

How else are the ara and gal operons regulated?

Answer 21

Both operons also have a CRP-binding site to regulate expression in response to [glucose].

Question 22

What does transcriptional attenuation often regulate?

Answer 22

Often involved in regulating amino acid biosynthesis.

Question 23

What operon is the model for transcriptional attenuation regulation?

Answer 23

The trp operon.

Question 24

What is the protein that partially regulates trp?

Answer 24

TrpR is a repressor protein that binds to trpO in the presence of tryptophan.

Question 25

When is TrpR inactive?

Answer 25

Inactive when tryptophan is ABSENT.

Question 26

What else is trp regulated by, other than TrpR?

Answer 26

Also regulated by RNA secondary structure of trpL.

Question 27

What happens to trpL when [tryptophan] is high?

Answer 27

An attenuator sequence forms; an attenuator sequence is a hairpin loop directly adjacent to a string of A:U pairs.

Question 28

What does the attenuator sequence when [tryptophan] is high cause?

Answer 28

Hairpin causes RNApol to pause, and the A:U pairs cause instability and the RNA pol falls off (called a terminator loop).

Question 29

What happens to trpL when [tryptophan] is low?

Answer 29

The ribosome stops at region 1 of trpL.

Question 30

What is the mechanism of trpL when [tryptophan] is low? (4)

Answer 30

1. there are two UGG codons in region 1 causing the ribosome to stop because there is no tryptophan 2. ribosome blocks region 1 from forming a hairpin with region 2 3. a 2:3 hairpin forms away from the poly(U) region = antiterminator loop 4. this prevents the attenuator hairpin from forming and transcription continues.

Question 31

Why is the attenuator regulation of trp only in prokaryotes?

Answer 31

Eukaryotes don't have coupled transcription and translation.

Question 32

What is global regulation?

Answer 32

The coordinated expression of a variety of genes throughout the chromosome.

Question 33

What is a bacterial example of global regulation?

Answer 33

SOS response which occurs if there is extensive DNA damage.

Question 34

What does the SOS response involve (2) and do?

Answer 34

It involves RecA and the LexA repressor. It increases expression of trans-lesion synthesis polymerases that replicate across DNA lesions.

Question 35

What is the SOS response mechanism? (4)

Answer 35

1. LexA normally binds SOS genes to inhibit their expression 2. DNA damage leads to gaps in the DNA to which RecA binds 3. RecA bound to DNA causes self-cleavage of LexA 4. SOS genes are expressed

Question 36

What are riboswitches?

Answer 36

A mechanism of post-translational regulation. They are mRNA that interact with compounds and affect the completion of transcription or translation of that mRNA. (conf. Δ!)

Question 37

What are translational repressors?

Answer 37

A mechanism of post-translational regulation. They bind mRNA to prevent translation.

Question 38

What is an example of a translational repressor?

Answer 38

r-protein translation being blocked when there is too little rRNA present in a cell.

Question 39

What is the stringent response?

Answer 39

A mechanism of post-translational regulation. When amino acid concentrations decrease, cells initiate the stringent response to push resources to amino acid synthesis.

Question 40

What is the mechanism of the stringent response? (4)

Answer 40

1. uncharged tRNAs bind the A sites 2. RelA (stringent factor) binds the ribosome 3. RelA catalyzes the formation of ppGpp 4. ppGpp binds RNApol and inhibits transcription of rRNA to reduce ribosome production and decrease translation rates