Control of gene expression in prokaryotes

Question 1

Define the initiation rate

Answer 1

number of mRNA molecule whose synthesis is initiated per unit of time

Question 2

What two factors impact the initiation rate

Answer 2

1. consensus sequences

2. Proteins that change the affinity of the RNA polymerase for the promoter

Question 3

What are the two consensus sequences in the promoter region

Answer 3

1. TATAA

2. TTGACA

Question 4

What are the two types of transcription control

Answer 4

Positive and negative control

Question 5

Define positive control

Answer 5

Genes under positive control will only be expressed when an activator protein is present

Question 6

Define Negative control

Answer 6

Genes under negative control are expressed unless a repressor protein is present to block their transcription

Question 7

Define an inducer (in context of repressors)

Answer 7

A ligand that binds to a repressor protein, causing it to detach

Question 8

Define a co-repressor

Answer 8

A ligand that is needed to help bind repressor to DNA

Question 9

Define an inducer (in context of activators)

Answer 9

It binds the activator, allowing it to bind DNA and machinery

Question 10

Define catabolic inhibition

Answer 10

The process by which genes coding for certain catabolic machinery is turned off when not needed.

Question 11

What are the two main components of the lac operon?

Answer 11

1. cis-acting regulator elements

2. protein-coding structural genes

Question 12

What are the four products of the lac operon?

Answer 12

LacI
Lac Z
Lac Y
LacA

Question 13

Define the lac Z gene

Answer 13

codes for β-galactosidase, the enzyme that breaks the glycoside bond in
lactose

Question 14

Define the lacY gene

Answer 14

codes for lactose permease, the protein that transports lactose in the cell

Question 15

Define the lacA gene

Answer 15

Codes for sulfogalactoside transacetylase; this enzyme plays a role detoxification of
thiogalactosides that are also transported into the cell by the lacY gene product.

Question 16

What type of control is the lactose repressor an example of?

Answer 16

Negative control:

- controlled by both a repressor and an activator

Question 17

Define the repressor of the lactose operon

Answer 17

1. coded in the lacI gene
2. binds to the operator sequence, which overlaps with the start sequence, inhibiting it
3. Forms a tetramer

Question 18

Define allolactose

Answer 18

1. Intermediate of lactose metabolism

2. binds to repressor protein, causing it to dissociate from the DNA

Question 19

What kind of control is demonstrated in the activator of the lactose operon?

Answer 19

Positive control

Question 20

What is the activator for the lactose operon?

Answer 20

CAP (catabolic activator protein)

-acts as a dimer, increases affinity of RNA pol. for promoter

Question 21

cAMP must be present for CAP to bind DNA. What influences the relative presence of cAMP?

Answer 21

intracellular concentrations of glucose. As glucose conc. goes up, cAMP conc. goes down.

Question 22

Status of operon when glucose and lactose present

Answer 22

Operon is off, because glucose inhibits cAMP, therby preventing the binding of CAP to DNA

Question 23

Status of operon when only glucose present

Answer 23

repressor is bound, due to absence of allolactose. Therefore, operon is off

Question 24

Status of operon when both lactose and glucose are absent

Answer 24

operon is off, lac repressor is still bound. However, CAP is bound as well, since absence of glucose allows presence of cAMP

Question 25

Status of operon when only lactose present

Answer 25

CAP binds, allolactose removes repressor, operator is ON