13.3 Lac Operon

Question 1

What is the Lac operon used as an example of?

Answer 1

Transcriptional gene regulation

Question 2

What is gene expression?

Answer 2

The process that heritable information in a gene is made into a functional gene product, such as protein or RNA

Question 3

What is gene regulation?

Answer 3

The process of controlling gene expression

Question 4

When can the mechanisms of gene regulation occur?

Answer 4

during any part of gene expression, including transription, RNA processing, translation, and post-translational modifications

Question 5

What is RNA processing in eukaryotes?

Answer 5

• series of modfications a newly transcribed RNA molecule undergoes before it can be used for protein synthesi
• includes splicing out introns

Question 6

What are post-translational modifications?

Answer 6

Messenger RNAs are translated into proteins. Sometimes proteins are nonfunctional when first translated, and need to be modified into functional proteins

Question 7

In bacteria, are translation and transcription coupled?

Answer 7

yes

Question 8

When is regulating gene expression mainly done in bacteria?

Answer 8

At the transcriptional level

Question 9

What is the preferred energy source for bacteria?

Answer 9

Glucose

Question 10

What happens when glucose isn’t present?

Answer 10

Alternative energy sources can be utilized, such as lactose

Question 11

What is lactose converted to in e coli?

Answer 11

• galactose and glucose
• A small amount is converted into allolactose

Question 12

If there is glucose around, will bacteria make the enzymes required to metabolize lactose?

Answer 12

• No, because using glucose is more efficient
• lactose pathway will only be turned on when lactose is present

Question 13

What is an open reading frame?

Answer 13

A section of a DNA or RNA sequence that, if translated, has the potential to produce a protein

Question 14

What is an operon?

Answer 14

Multiple coding sequences on the same mRNA

Question 15

What comprises the lac operon?

Answer 15

• 3 open reading frames, lacZ, lacY, and lacA that are transcribed together on the same mRNA

Question 16

Is the promoter sequence located upstream or downstream of the lac Z gene?

Answer 16

upstream

Question 17

Where is the +1 transcription start site located?

Answer 17

Just downstream of the promoter sequence

Question 18

What does the lacZ gene do?

Answer 18

encodes for the protein known as B-galactosidase

Question 19

What does B-galactosidase do?

Answer 19

Is is the enzyme that converts lactose to allolactose and to galactose and glucose

Question 20

What does lac Y encode for?

Answer 20

Lactose permease

Question 21

What does lactose permease do?

Answer 21

The protein that allows lactose to enter the cell

Question 22

What does the lac A do?

Answer 22

• unknown
• knocking the gene out doesn’t seem to have an effect

Question 23

What is lac I? What does it do?

Answer 23

A regulatory gene that encodes for the lac repressor

Question 24

Where is lacI located?

Answer 24

upstream of the lac operon

Question 25

Is lacI part of the lac operon?

Answer 25

* No, not officially * It has it's own promoter sequence and creates its own independent transcript

Question 26

What does cap stand for? Where is it located?

Answer 26

* makes CATabolite activator protein * located elsewhere on circular chromosome of e coli, has its' own promoter sequence

Question 27

Are CAP and Lac repressor expressed consistently? What is this called?

Answer 27

* Yes * This is called constitutive gene expression

Question 28

What is constitutive gene expression?

Answer 28

the genes are expressed all of the time, at constant levels

Question 29

Where is the operator located?

Answer 29

between the promoter and lac operon

Question 30

What is the operator?

Answer 30

The domain that the lac repressor can bind to

Question 31

Where is the CAP binding site?

Answer 31

upstream of the promoter (but downstream of lacI)

Question 32

What happens when lac repressor binds to the operator sequence?

Answer 32

* It physically gets in the way of RNA polymerase which is trying to sit on the promoter to induce gene expression * So, when the lac repressor is bound, transcription is blocked

Question 33

What happens when lactose is present?

Answer 33

* A small amount of lactose is converted into allolactose * allolactose can bind to the lac repressor

Question 34

What happens when allolactose binds to the lac repressor?

Answer 34

* it prevents the lac repressor from binding to the operator sequence * now, RNA polymerase can bind to the promoter and begin transcription

Question 35

What happens when there is no lactose present?

Answer 35

the lac repressor binds, prevents the +1 site from being accessible to RNA polymerase... there is no transcription

Question 36

What happens when there is lactose present?

Answer 36

allolactose binds to the lac repressor and induces transcription

Question 37

What type of regulation is the lac repressor?

Answer 37

negative regulation

Question 38

What is negative regulation?

Answer 38

When the binding of a regulatory protein blocks transcription

Question 39

What is an inducer?

Answer 39

A small molecule that activates gene expression | allolactose and cAMP are inducers

Question 40

What happens when cAMP binds to CAP?

Answer 40

* CAP is inactive, but when cAMP binds it becomes active and can bind to the CAPsite

Question 41

What happens when CAP/cAMP binds to the CAP site?

Answer 41

RNA polymerase is recruited to the promoter, resulting in high levels of transcription

Question 42

What is cAMP?

Answer 42

* a signaling molecule produced by glucose metabolism

Question 43

When glucose is low, cAMP is...

Answer 43

high

Question 44

Glucose, cAMP, and CAP relationship

Answer 44

* when glucose levels are high, cAMP is low. cAMP doesn't bind to CAP, and CAP remains inactive * When glucose levels are low, cAMP is high. cAMP will bind to CAP, and CAP will bind to the CAP binding site. This recruits DNA polymerase to the promoter sequence, and results in high transcription levels.

Question 45

What happens to DNA when CAP binds to the CAP binding site?

Answer 45

* The DNA loops around in such a way that the -10 and -35 sites become more accessible to RNA polymerase * RNA poly is recruited to the promoter site, inducing very high levels of transcription

Question 46

What is positive regulation?

Answer 46

When binding of a regulatory protein activates transcription | CAP is an example of positive regulation

Question 47

What happens when there is lactose but no glucose

Answer 47

* With lactose is present, there is allolactose * Allolactose binds to the lac repressor, and prevents the lac represser from binding to the operator * Highest levels of transcription (also because CAP is binding, bc low glucose=high cAMP= CAP is active and can bind)

Question 48

What happens when there is no lactose, but there is glucose?

Answer 48

* If there is no lactose (no allolactose) the lac repressor binds to the operator * High glucose= low cAMP (no CAP binding) * Absolutely no transcription

Question 49

What happens when there is lactose and glucose present?

Answer 49

* high glucose = low cAMP = no CAP binding * lactose present=allolactose present= lactose repressor does not bind to operator * Some transcription occurs because RNA poly can find promoter sequence

Question 50

What happens when there is no lactose, and no glucose?

Answer 50

* no lactose= no allolactose. Lactose repressor binds to the operator site * low glucose= high cAMP. CAP binds and poly recruited * even though polymerase is recruited, transcription is still blocked. * There is virtually no transcription (every now and then there will be a transcript)... aka leaky transcription