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Transcription Flashcards

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Biology 101 flashcards
1
Q

What are transcription factors

A

Proteins that regulate transcription

Have DNA binding domain and transactivating domain

~10% genes code for transcription factors

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2
Q

Examples of transcription factors

A

Helix-turn-helix - In all organisms

Zinc finger domain - Common in humans

Coiled coil domains - In leucine zipper and helix-loop-helix motifs

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3
Q

What are activators and repressors?

A

Activators are proteins that increase transcription

Repressors are proteins that decrease transcription

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4
Q

What do activators and repressors bind to?

A

Regulatory DNA sequences called operators

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5
Q

What can RNA polymerase holoenzyme always transcribe?

A

Any gene with a functional promoter

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6
Q

Where can enhancers be found?

A

Up or downstream of gene

Enhancers - increase transcription
Silencers - decrease transcription

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7
Q

What genes control gene expression in bacteria?

A

Constitutive and regulated genes

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8
Q

Constitutive genes

A

Encode proteins cell always needs

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9
Q

Regulated genes

A

Expression only required under certain circumstances

Expression in bacteria regulated at transcription level

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10
Q

Operons

A

Genes expressed to encode proteins that work together is co-ordinated by organisation of genes into operons

Operon genes transcribed together to one polycistronic mRNA - translated into separate proteins

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11
Q

What occurs when lactose added to sole carbon source in E. coli?

A
  • Increase in lac mRNA transcript
  • Rapid synthesis of lactose-metabolizing enzymes

Beta-galactosidase hydrolyses lactose to glucose and galactose

Lactose converted to allolactose

Permease enables lactose to enter cell

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12
Q

How are lac genes expressed

A

lac operon (lacZ, lacY, lacA) transcribed to produce mRNA

mRNA forms beta-galactosidase (lacZ), Permease (lacY), Transacetylase (lacA)

lacI upstream of operon - express lac repressor and has own promoter

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13
Q

When does lacI bind lac operator

A

When no lactose is present

RNA pol can’t bind promoter

No transcription

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14
Q

What occurs with lacI when lactose present

A

allolactose binds lacI, altering shape

Transcription occurs

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15
Q

What occurs in transcription when glucose present

A
  • Inactivates adenylyl cyclase
  • Low cAMP levels

cAMP indicates glucose levels

cAMP high = low glucose

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16
Q

What is CAP protein?

A

Catabolite activator protein

CAP dimerizes in cAMP presence

Dimerized CAP binds DNA

Bends DNA by 90 degrees

17
Q

What occurs when CAP-cAMP binds CAP operator?

A

RNA polymerase holoenzyme binds promoter

Activates more then 100 promoters in E. coli in low glucose levels

18
Q

Name 2 molecules that combine lac operon regulation

A

lac repressors and CAP-cAMP

19
Q

Explain mRNA in tryptophan biosynthesis

A

5 genes encoding enzymes expressed in operon as polycistronic mRNA

20
Q

When is trp operon turned off

A

excess tryptophan

Regulatory gene is trpR

21
Q

Tryptophan-mediated repression

A

trpR encodes aporepressor

No binding of TrpR aporepressor in absence of tryptophan

Aporepressor activated by binding tryptophan in excess tryptophan

Binds operator, prevents transcription

Regulation acts as on/off switch

22
Q

Explain second regulatory mechanism of trp operon

A

Tryptophan starvation - Maximal expression of trp genes

tryptophan limited - less than maximal expression of trp genes

Attenuation produces short transcripts that don’t encode structural proteins

Termination at attenuator site with trpL leader region of operon

Attenuation is based on fact bacterial transcription coupled to translation

23
Q

trp operon regulation at initiation

A

Absence of trp - No repression - transcription

Presence of excess trp - TrpR aporepressor + tryptophan co-repressor bind operator - No transcription

24
Q

What are riboswitches?

A

Portion of transcript

Bind small molecule that controls RNA secondary structure, regulating transcription/translation

25
Q

2 regions of riboswitch

A

Aptamer - Binds metabolite

Expression platform - control transcription or translation

26
Q

Adenine riboswitch

A
  • Regulates adenine synthesis and transport in B. subtilis
27
Q

What does gene expression depend on with adenine riboswitches?

A

Whether terminator or anti-terminator forms

Low adenine - regions 2 and 3 form anti-terminator, prevents regions 3 and 4 forming terminator

High adenine - adenine binds aptamer leading to regions 3 and 4 pairing and forming terminator

Genetics (18 decks)

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