Ch. 11 Regulation Of Gene Expression In Bacteria

Front 1

At what levels can gene expression be regulated?

Back 1

1. Transcriptional level (no transcription)

2. Translational level (no protein)

3. Posttranslational level (proteins regulated)

Front 2

What are the two categories of genes?

Back 2

Constitutive (always on because always needed) and regulated (can be turned off or on as needed by the cell).

Front 3

What does transcriptional regulation do?

Back 3

Increases OR decreases the rate of transcription. It involves specific DNA sequences, proteins, and small organic molecules.

Front 4

What are the two regulatory proteins?

Back 4

Activators (increase transcription rate. Called positive control).

Repressors (decrease transcription rate. Called negative control).

Front 5

What is positive regulation?

Back 5

When a gene requires an activator protein to bind to its target DNA site for transcription to begin.

Front 6

What is negative regulation?

Back 6

When a gene requires a repressor protein to be prevented from binding to its target site so transcription can begin.

Front 7

What are allosteric effectors?

Back 7

Small organic molecules that bind to allosteric sites of regulatory proteins to change their shapes so they either can or can't bind to DNA.

Activators: Bound so they CAN work

Repressors: Bound so they CAN'T work.

(This is true of inducers. Inhibitors and corepressors work differently).

Front 8

What are the three different types of allosteric effectors?

Back 8

1. Inducers. Increase the rate of gene transcription by binding to a repressor or activator.

2. Corepressors: Decrease transcription rates by binding to a repressor, causing it to bind to DNA.

3. Inhibitors: Decrease transcription rate by binding to activators, preventing them from binding to DNA.

Front 9

What is an inducible gene?

Back 9

One that is always "off" until it is turned on.

Front 10

What is a repressible gene?

Back 10

One that is always "on" until turned off.

Front 11

What are the four combinations of regulatory proteins, allosteric effectors, and gene types?

Back 11

1. Repressor protein, inducer molecule, inducible gene: Gene turned on.

2. Activator protein, inducer molecule, inducible gene: Gene turned on.

3. Repressor protein, corepressor molecule, repressible gene: Gene turned off.

4. Activator protein, inhibitor molecule, repressible gene: Gene turned off.

Front 12

What is an operon?

Back 12

A set of adjacent structural genes whose mRNA is synthesized in one piece (multigenic mRNA). Under the transcriptional control of a promoter and a regulatory region.

Front 13

What is multigenic mRNA?

Back 13

It codes for multiple proteins. These proteins are usually involved in a particular metabolic pathway.

Front 14

What is the DNA Binding Domain?

Back 14

The site on a DNA-binding protein that directly interacts with specific DNA sequences.

Front 15

Describe the lac operon.

Back 15

Front 16

Is the lac operon inducible or repressible? What does that mean?

Back 16

It is inducible. This means it is off until presence of an allosteric effector binds to a repressor to turn the operon on.

Front 17

Is the trp operon inducible or repressible? What does this mean?

Back 17

Repressible. It is always on unless a corepressor binds to the repressor to allow the repressor to bind to the operon and turn off transcription.

Front 18

What are the following: lacA, lacI, lacO, lacP, lacZ, and lacY?

Back 18

1. Encodes galactoside transacetylase. Hypothesis is it modifies lactose to prevent toxic build up.

2. Encodes the lac repressor protein

3. The operator. Where the lac repressor binds.

4. Promoter. Where RNA Polymerase binds.

5. Encodes beta galactosidase: breaks lactose down into glucose and galactose. Converts some lactose into allolactose inducer.

6. Encodes lactose permease. Helps import lactose into cell.

Front 19

What are constitutive mutations?

Back 19

Changes in DNA sequences that cause genes that are repressed at times to be expressed continuously. (Lac O^c mutation and I- mutation).

Front 20

What are cis-acting mutations?

Back 20

Changes in the DNA operator sequence. Repressor proteins can no longer bind to the operator, so the lac operon (for example) is always on. Only affects sequences on the same chromosome. (The O^c mutation, which is non-induced).

Front 21

What are trans-acting mutations?

Back 21

Mutations in the repressor protein gene. Produces repressors that cannot bind to the operon. (I- mutation. It is recessive to the I+ wild type, so if heterozygous, good repressors will still bind to all operators, even if they aren't on the same chromosome. Non-induced).

Front 22

What are supressor I^s mutations?

Back 22

They are dominant over I+, unlike I-. They create super repressors that are unaffected by inducers. Their allosteric sites are altered, so they can't bind inducers. Even the presence of normal repressors won't allow the operon to turn on because the inducers can't turn off the super repressors.

Front 23

What is positive control of the lac operon?

Back 23

Catabolite repression. It is the inactivation of the operon caused by the presence of glucose (preferred by bacterial cell over lactose because it is a breakdown product/catabolite of lactose) as well as lactose. It is positive control because it relies on an activator protein instead of a repressor protein.

Front 24

Describe how the lac operon is regulated when both glucose and lactose are present.

Back 24

When glucose concentrations are high, the production of allosteric effector cAMP (cyclic adenosine monophosphate) is inhibited. This prevents it from binding to the activator protein CAP (catabolite activator protein), preventing CAP from activating the lac operon.

However, if glucose is low, then cAMP will be produced, bind to CAP, and CAP will bind to the lac promoter to facilitate RNA polymerase.

Front 25

Where do CAP and RNA polymerase bind in relation to each other? Why?

Back 25

Next to each other so CAP can interact with RNA Polymerase to facilitate lac operon transcription.

Front 26

Can the lac operon be controlled both negatively and positively? How?

Back 26

Yes. Negative control requires the inactivation of a repressor protein by an inducer such as allolactose so it can't bind to the operator, which allows transcription.

Positive control requires an activator like CAP to be bound by an allosteric effector like cAMP so it can bind to the operator and allow transcription.

Front 27

What are the two mechanisms that control the tryptophan operon?

Back 27

1. The trpR gene codes for the Trp repressor, which binds to the operator and prevents transcription. In this case, trp acts as the corepressor for the Trp repressor.

2. Attenuation

Front 28

When tryptophan is absent, what happens to the trp operon?

Back 28

It is turned on due to an inactive repressor because of the absence of corepressor.

Front 29

What happens to the trp operon when trp is present?

Back 29

It is turned off via a repressor that functions because it is bound by its corepressor, tryptophan

Front 30

What is attenuation?

Back 30

A regulatory mechanism (such as with the trp operon) in which the level of transcription of an operon is reduced when the end product of the pathway (such as tryptophan) is plentiful.

Front 31

What is an attenuator?

Back 31

A region of RNA sequence that forms alternative secondary structures that govern the level of transcription of attenuated operons.

Front 32

How does attenuation of the trp operon work?

Back 32

1. The order of genes is trpR protein repressor, promoter, operator, trpL (leader peptide, 14 aa long), attenuation sequence, and then the structural genes.

2. Transcription of the DNA is terminated as soon as it hits the attenuation sequence. Occurs when trp levels are high. Operon turned off.

Front 33

What are the three possible trpL stem loop structures?

Back 33

1. When translation is not coupled with transcription, regions 1 and 2 of the trpL mRNA bond as do regions 3 and 4. A terminator stem loop forms, and transcription is terminated just past the trpL gene. Rho-independent (intrinsic) termination.

2. When coupled transcription and translation occur but trp levels are very low. Ribosome pauses at trp codons in trpL gene due to insufficient charged tRNA^trp. Blocks mRNA region 1 so 2 and 3 can bond. The 3 and 4 stem loop can't form. In this case, transcription is not terminated.

3. Coupled transcription/translation with high levels of trp. Translation of trpL gene progresses until its stop codon. Ribosome pauses, blocking region 2. Regions 3 and 4 bond, terminating transcription at U-rich attenuator.

Front 34

What are two features important to attenuation?

Back 34

1. The trpL mRNA contains 2 trp codons that act as sensors to determine if a cell has sufficient Trp to synthesize its proteins.

2. It also has the ability to form a stem-loop structure.