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MBG 2040 > gene expression > Flashcards

gene expression Flashcards

1
Q

regulation of gene expression in bacteria

A
  • a bacterium adapts depending on the environment
  • genes that are not required are not expressed unless environmental conditions change (regulation is very tight)
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2
Q

levels of gene regulation

A
  • alteration of structure
  • transcription
  • mRNA processing
  • RNA stability
  • Translation
    -post-translational modification
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3
Q

constitutively expressed genes

A

genes that specify products that are essential components of living cells
- need all the time and are always turned on

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4
Q

inducible and repressible genes

A
  • needed for cell growth or only under certain environmental conditions
  • regulatory mechanisms allow the synthesis of these gene products only when they are needed
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5
Q

ligand

A

binds to a repressor activator and tells it these are the conditions under which you should bind to the DNA

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6
Q

Negative control: operon types

A

Repressible: default mode is on
inducible: default mode is off

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7
Q

Negative control: operon types

A

Repressible: default mode is on
inducible: default mode is off

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8
Q

positive regulation in bacteria

A
  • involves an activator which when bound allows transcription of mRNA
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9
Q

positive inducible genes

A
  • usually off (activator not bound)
  • in absence of ligand binding to activator, gene is turned off
  • if ligand binds to activator, activator will bind to promotor and transcription will happen
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10
Q

positive repressible genes

A
  • usually off (activator bound)
  • when ligand binds to activator it leaves the promoter and transcription no longer happens
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11
Q

negative regulation in bacteria

A
  • repressor is involved which when bound prevents transcription of mRNA
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12
Q

negative repressible genes

A
  • usually on (repressor not bound)
  • when ligand binds to repressor, repressor binds and strops transcription
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13
Q

negative inducible

A
  • usually off (repressor bound)
  • when ligand binds to repressor it leaves and transcription can happen
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14
Q

Positive control: CAMP and CRP

A
  • low glucose causes buildup of cyclic AMP (cAMP)
  • camp binds to regulatory protein CRP
  • this activates the protein which binds to promoter of the operon
  • allows RNA polymerase to bind and begin transcription (turns on)
  • as glucose levels rise cAMP levels lower, decreasing rate of transcription
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15
Q

induction of genes for lactose utilization

A
  • for utilization of lactose gene expression is induced when lactose is present and glucose is absent
  • alters the rate of lactose-metabolizing enzyme synthesis
  • enzymes involved in catabolic pathway are often inducible (off)
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16
Q

repression of genes for tryptophan biosynthesis

A
  • genes are expressed in the absence of tryptophan and turned off when tryptophan is available
  • enzymes involved in anabolic pathways are often repressible (on)
17
Q

operon

A

a series of related genes that are expressed coordinately through the use of a single promoter and operator
- most structural genes with related functions in bacteria are organized into operons for efficiency of gene expression

18
Q

negative inducible operons

A
  • usually off
  • the regulator protein is active and bound to the operator and prevents transcription of the structural genes
  • then the inducer is present it binds to the regulator making it unable to bind to the operator and transcription takes place
19
Q

inducer

A

ligand that helps to express an operon
- may bind to repressor to take it off or bind to activator to put it on

20
Q

negative repressible operons

A
  • usually on
  • regulator protein is an inactive repressor, unable to bind to operator, transcription takes place
  • levels of corepressor build up which bind to regulator protein making it active and able to bind to operator, preventing transcription
21
Q

positive control of gene expression

A

the regulatory protein “activator” binds to DNA and interacts with RNA polymerase to assist the efficiency or transcription of structural genes

22
Q

The lac operon of E.coli

A
  • makes lactase to break down lactose
  • genes are transcribed only when lactose is present and glucose is absent
  • consists of regulatory repressor “I” gene (contains its own promoter), a second promoter “P”, a regulatory operator “O”, and 3 structural genes (Z,Y,A)
23
Q

operator

A
  • a piece of sequence that sits after the promoter and binds to repressor
24
Q

lac operon system

A
  • lactose premease (Y gene): lactose transporter (in membrane)
  • B-galactosidase (Z gene): main purpose is to turn lactose into glucose and galactose, low background level converts lactose (allolactose - lac operon inducer)
  • lac I region: encodes lac repressor, diffusible tetramer that represses in the absence of all lactose
  • lac O region: the lac operator, regulatory DNA sequence that works in cis to control lac operon, binding site
25
Q

lac operator

A
  • overlaps the initiation start site
  • when bound by lac repressor RNA poly can’t access promoter
26
Q

when is the lac operon ON or OFF

A

ON: in the presence of allolactose
- lac repressor binds inducer and is inactive
- induction results in transcription of lac Z, Y, and A genes
OFF: in the absence of the inducer allolactose
- the lac repressor binds to the lac operator and prevents RNA poly from transcribing Z, Y and A

27
Q

what happens when there is a mutation in the lac operator

A
  • genes are always on
28
Q

constitutive lac mutants

A
  • caused by mutations in the I gene (I-) or the operator (Oc)
  • Oc mutations act only in cis
  • wild type Z+, Y+ and A+ are dominant (from functional enzymes) and I+ is dominant
29
Q

lacl superrepressors

A
  • some lac mutants were never inactivated by allolactose
  • in these mutants the allolactose binding site in the lac repressor gene is altered
  • lacl repressor is always bound to the operator, preventing transcription of genes
  • in a diploid, Lacls is dominant over lacl+
30
Q

Is the lac operon always induced in the presence of allolactose

A
  • no, not if glucose is also present
  • glucose prevents lac operon induction- negative regulation
  • aka catabolite repression
31
Q

how does glucose regulate the lac operon

A
  • indirectly through cAMP levels
  • when glucose is high, cAMP is low
  • when cAMP is high (low glucose), it binds to CAP which permits binding to a special region in the lac promoter - required for lac operon transcription
32
Q

positive control of the lac operon (low glucose)

A

1) cAMP/CAP binding
2) RNA poly binding
- cAMP/CAP orients RNA poly and associated sigma factor to start transcription

MBG 2040 (23 decks)

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