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Genetics - Heather > Lecture 3 > Flashcards

Lecture 3 Flashcards

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1
Q

Give an example of a catabolic operon

A

Lac operon

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2
Q

Give an example of a biosynthetic operon

A

Trp operon

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3
Q

What are biosynthetic operons involved in?

A

The expression of enzymes that synthesise small molecules (amino acids, nucleotides). If these are not present in the growth media the operon is switched on.

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4
Q

Charles Yanofsky

A

Discovered the trp operon

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5
Q

What are the 5 structural genes in the trp operon?
&
What do they encode?

A

trpA, tepB, tepC, trpD, trpE

The encode 5 enzymes in the biosynthetic pathway that leads to the production of a tryptophan amino acid

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6
Q

Which gene codes the repressor for the trp operon?

A

trpR

Works in transT

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7
Q

Trp operon: apo-repressor

A

Trp repressor

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8
Q

Trp operon: holo-repressor

A

Trp repressor & tryptophan

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9
Q

How does tryptophan control its own expression?

A

When there are high concentrations of tryptophan it binds the trp repressor to form the holo-repressor.
When tryptophan concentration is low tryptophan falls off the trp repressor.

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10
Q

When are biosynthetic operons off?

A

When the molecule they help synthesise is available

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11
Q

What is positive control?

A

The activator molecule binds to an operator region & transcription occurs.
Repression occurs when you inactivate the activator molecule
e.g. CAP proteins

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12
Q

What is negative control?

A

The repressor molecule binds to an operator region to stop transcription.

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13
Q

What is the arabinose operon an example of?

A

Dual control- protein with repressor & activation activity

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14
Q

What is the order of genes in the arabinose operon?

A

C O I B A D

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15
Q

C gene in the arabinose operon

A

ACTIVATOR PROTEIN

control gene

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16
Q

O & I genes in the arabinose operon

A

AraO and AraI are the control sites

INITIATOR REGION

17
Q

B & A & D genes in the arabinose operon

A 
STRUCTURAL GENES 
B = kinase
A = isomerase
D = epimerase 
together these convert the sugar arabinose to D-xylase-5-phosphate
18
Q

What happens in the presence of arabinose?

A

The operon needs to be switched on to provide enzymes involved in arabinose utilisation.
CAP + cAMP bind
AraC + arabinose bind
These act as an activator to promote transcription

19
Q

What happens if arabinose is absent?

A

AraC undergoes a conformational change so it can bind the O & I region. Operon loops to allow this binding and transcription is obstructed

20
Q

What are the two main methods for controlling transcription?

A

Choose which genes to start transcribing:
+ alternative sigma factors
+operons: co-ordinated control of gene groups

Deciding where to stop transcribing:
+anti-terminators
+attenuation

21
Q

Anti-terminators

A

Anti-termination sites have an anti-terminator protein so RNA polymerase will pick up the anti-terminator protein as it moves along the chromosome. This allows RNA polymerase to pass termination signals and transcribe subsequent genes.

22
Q

What is attenuation?

A

A regulatory mechanism for the Tryptophan operon and is focused on the leader sequence.

23
Q

Yanofsky & attenuation

A

Saw that bacterium with a deletion mutation in the Trp operon would produce more Trp operon transcripts. Transcripts were longer in these mutants.
The mutation was a deletion at the end of the leader region which was a terminator sequence.

24
Q

Describe the trpL leader region

A

146 bp and has 3 regions of inverted repeats

25
Q

Describe the inverted repeats in trpL

A

Only 1/3 of the inverted repeats is a functional terminator as it has a run of AAA/UUU. The other inverted repeats only result in stem loop formation.
The inverted repeat regions overlap.

If stem loop 1 and 3 form termination occurs as stem loop 3 is the only loop with the AAA/UUU

If stem loop 2 forms termination does not occur.

26
Q

Transcription/translation when high tryptophan

A

RNA polymerase is transcribing the leader region and transcribes the first part of the RNA that contributes to the formation of stem loop 1. The ribosome moves along behind the DNA polymerase.

RNA polymerase transcribes the region that corresponds to stem loop 2.

RNA polymerase completes transcribing the section that corresponds to stem loop 2. The ribosome finishes and dissociates. Stem loop 1 forms folding in on itself

Transcription stops as termination

27
Q

Transcription/translation when low tryptopha

A

The ribosome is following RNA polymerase

In stem loop 1 the ribosome needs 2 tryptophans so the ribosome stalls and does not dissociate.

The polymerase has completed transcribing stem loop 1 but it cannot fold as the ribosome is there. RNA polymerase completed transcribing stem loop 2.

Transcription continues as no termination

Genetics - Heather (8 decks)

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