18 Gene Expression in Bacteria

Question 0

Cells a very selective

Answer 0

• about genes they express
• how strongly they are expressed
• when they are expressed

Question 1

Gene expression

Answer 1

A gene product is actively being synthesized
Efficient use of resources
Survival

Question 2

Information flow

Answer 2

DNA —> RNA —> Protein —> activated protein

Question 3

Transcriptional control

Answer 3

When the cell does not production mRNA for specific enzymes

By utilizing regulatory protein
Preventing RNA polymerase from binding to

Question 4

Translational control

Answer 4

Allows cell to prevent the translation of an mRNA molecule that has already been transcribed

Regulatory molecules can speed up mRNA degradation

Question 5

Speed and efficiency of three controls

Answer 5

• translational slow but efficient
• transcription faster but more resource
• post-transcription fastest but most energetic

Question 6

What is lactose made of?

Answer 6

Lactose is a disaccharide made up of one molecule of glucose and one molecule of galactose

Question 7

What enzyme breaks down lactose?

Answer 7

β-galactosidase - into glucose and galactose

Question 8

Where does the glucose from lactose go to after lactose breakdown?

Answer 8

Into the glycolytic pathway

Question 9

What does the cell do with the galactose?

Answer 9

Other enzymes convert galactose to a substance that can also be processed in the glycolytic pathway

Question 10

What two scientist introduced the lactose metabolism in E. coli as a model system?

Answer 10

Jacque Monod and François Jacob

Question 11

What is an inducer?

Answer 11

An inducer is a small molecule that triggers transcription of a specific gene

Question 12

What does lactose induce in E. coli?

Answer 12

Lactose is an inducer for the β-galactosidase gene

Question 13

Does E. coli produce β-galactosidase when both glucose and lactose are present.

Answer 13

No - only produced when lactose is present with not glucose

Question 14

What is a mutagen?

Answer 14

X-rays, UV light, chemicals that damage DNA and increase mutation rates.

Question 15

What two steps were taken to find the particular trait desired for research in lactose metabolism?

Answer 15

1. Generate a large number of individuals with mutations at random locations in their genomes. Used mutagen
2. Screen the treated individuals for mutants with defects in the process or biochemical pathway in question - in this case, defects in lactose metabolism

Question 16

Steps in Replica Plating to find Lactose Metabolism Mutants

Answer 16

1. Mutants with defects in lactose metabolism are desired, mutagenized bacteria are spread on a “master plate” filled with a gelatinous growth medium containing glucose but not lactose
2. A block covered with a piece of sterilized velvet is pressed onto the master plate. Some cells from each colony on the mater plate are transferred to the velvet.
3. The velvet is pressed onto a plate called a replica plate that contains medium that differs from the master plate by a single component. In this case, the second medium has only lactose. Cells from the velvet stick to the replica plate’s surface, producing an exact copy of the location of the colonies on the master plate.
4. After the transferred cells grow, compare the colonies on the replica plate with those on the master plate. In this example, colonies that grow on the master plate but are missing on the replica are mutant that cannot metabolize lactose.

Question 17

What are constitutive mutants?

Answer 17

Cells that are abnormal because they produce a product at all times.

Question 18

LacZ

Answer 18

Codes for β-galactosidase

Question 19

LacY

Answer 19

Codes for galactosidase permease

Question 20

LacI

Answer 20

Regulates gene expression

• in normal cells, when lactose is not present, lacI prevents the transcription of lacZ and lacY
• defective mutation on this genes causes the constitutive production of β-galactosidase and galactosidase permease.

Question 21

Negative control of transcription

Answer 21

Occurs when a regulatory protein called a depressor binds to DNA and shuts down transcription

Question 22

Positive control of transcription

Answer 22

Occurs when a regulatory protein called a activator binds to DNA and triggers transcription.

Question 23

What is an operon?

Answer 23

An operon is a set of coordinately regulated bacterial genes that are transcribed together into one mRNA.

Question 24

What is the name of the group of genes involved in lactose metabolism?

Answer 24

lac operon

Question 25

What is the function of lacA

Answer 25

Codes for enzyme transacetylase

Question 26

What is the function of transacetylase?

Answer 26

Enzyme that catalyzes the reactions that allow certain types of sugars to be exported from the cell when they are too abundant and could harm the cell.

Question 27

What are the three hypothesis of the Jacob-Monod model of lac operon regulation?

Answer 27

1. The lacZ, lacY, and lacA genes are adjacent and are transcribed into one mRNA initiated from the single promoter of the lac operon (cotranscription)
2. The repressor is a protein encoded by lacI that binds to DNA at the operator and prevents transcription of the lax operon genes.
3. The inducer (lactose) binds to the repressor. When it does, the repressor changes shape. The shape change causes the repressor to come off the DNA.

Question 28

What is the ara operon?

Answer 28

An operon that contains three genes that allow E. coli to use the sugar arabinose

Question 29

What is the function of AraC?

Answer 29

AraC is an activator protein that turns on the ara operon. Coded by the araC.

Question 30

How does AraC control the ara operon?

Answer 30

• The protein is allosterically regulated by arabinose.
• When bound to arabinose, two copies of the AraC protein attach to a regulatory sequence of DNA called the initiator.
• Initiator is upstream to the promotor.
• Once bounded it can also bind to RNA polymerase.
• Helps to dock polymerase to the promotor and accelerate the initiation of transcription.

Question 31

How does AraC act as a repressor?

Answer 31

• In the absence of arabinose
• AraC protein remain together
• one is bound to the initiator
• the second is bound to a different regulatory site called the ara operator.
• preventing transcription

Question 32

What is global gene regulation?

Answer 32

Is the coordinated regulation of many genes

Question 33

What is a regulon?

Answer 33

A set of separate genes or operons that contain the same regulatory sequences and that are controlled by a single type of regulatory protein.

Allow bacteria to respond to challenges that include shortage in nutrients, sudden changes in temperature, exposure to radiation, or shifts in habitat.

Question 34

SOS response regulon

Answer 34

Allows bacteria to repair extensive damage to DNA.

• damage sets off the SOS signal that induces the transcription of more than 40 genes (enzymes and polymerase)
• repair of DNA occurs.

Question 35

Uninduced state

Answer 35

Genes of the regulon silenced by common repressor

Question 36

Induced state

Answer 36

Repressor inactivated;

Regulon genes released for transcription and translation