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MBG 2040 > Week 12 > Flashcards

Week 12 Flashcards

1
Q

Why does genetic regulation occur in bacteria?

A

= environment often changes and genetic regulation is focused on adapting to this change
- genes that are not required generally are not expressed unless environmental change makes expression useful

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2
Q

At what levels can gene regulation occur in bacteria and eukaryotes?

A
  • alteration of structure: compactness of DNA (if compact, difficult to transcribe)
  • transcription: gene transcribed to make pre-mRNA, amounts and types of pre-mRNA can be regualted
  • esp. in eukaryotes processing of pre-mRNA
  • whether RNA is translated (regulated to translate later, or right away and quantity of translation)
  • protein might need post-transalational modification and this can be regulated to keep protein active or inactive
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3
Q

At which level is the lac operon in E coli regulated?

A
  • primarily at level of transcription
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4
Q

What are constitutively expressed genes?

A

= genes that specify products like tRNAs, rRNAs, ribosomal proteins, RNA polymerase subunits, enzymes catalyzing housekeeping functions etc
- these are essential components of almost all living cells and need to be continuously (=constitutively) expressed

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5
Q

What are inducible and repressible genes?

A

= gene products that are needed for cell growth only under certain env. conditions and regulatory mechanisms that allow synthesis of these gene products only when needed
- turned on = induced, shut off = repressed

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6
Q

Key facts about induction of genes for lactose utilization

A
  • glucose is preferred carbon source for bacteria
  • lactose used if nothing else available and need to induce enzymes involved in lactose utilization: happens when lactose present and glucose absent
  • induction at level of transcription: alters rate of lactose-metabolizing enzyme synthesis (before: low levels, when glucose absent, lactose present: levels go up)
  • enzymes involved in catabolic (breakdown) pathways are often inducible
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7
Q

Repression of genes for Trp biosynthesis

A
  • genes are expressed in absence of trp and turned off when trp is available
  • repression occurs at level of transcription of trp biosynthetic genes
  • enzymes involved in anabolic (synthesis) pathways are often repressible
  • so, if trp present, repress enzymes to synthesize it
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8
Q

What is a bacterial operon?

A

= single transcriptional unit that includes a series of structural genes, a promoter, and an operator = group of structural genes plus sequences that control transcription
- special regulator protein helps control expression of structural genes

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9
Q

How are most structural genes w related functions organized in bacteria?

A
  • most structural genes w related functions are organized into operons for efficiency of gene expression
  • this means that transcription of the set of genes in the operon is controlled by a single promoter and a regulatory sequence called “operator”
  • operators can bind regulatory proteins
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10
Q

What is a negative inducible operon?

A

= if NO inducer present: regulatory protein is a repressor biding operator and prevents transcription of the structural genes in the operon
= if inducer present: binds to regulator making it unable to to bind to the operator and transcription takes place
(lac operon works like this)

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11
Q

How does a negative repressible operon work?

A

= No product present: regulator protein is inactive repressor and unable to bind to operator, transcription of structural genes therefore takes palce
= Product present: the product binds to the regulatory protein and makes it active and able to bind to the operator, this prevents transcription

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12
Q

What is positive control of bacterial gene expression?

A

= regulatory protein is an activator, binds to DNA and interacts with RNA polymerase to assist in efficiency of transcription of structural genes
- lac operon is a negatively inducible but positively-regulated operon

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13
Q

What is co-ordinate expression?

A

= the fact that structural genes can be controlled as a single unit when they are grouped together in an operon

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14
Q

When are the genes of the lac operon transcribed?

A

= when lactose is present and glucose is absent

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15
Q

Where is the lac operon located?

A

b/n purE and proA genes on bacterial chromosome

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16
Q

What does the lac operon consist of?

A
  • lac repressor lacI gene w its own promoter - PI) = regulator gene
  • beta-galactosidase gene (lacZ)
  • beta galactoside transacetylase gene (lacA)
  • transcription of lacZ, Y and A genes controlled by single lac promoter (P) and lac operator (O) sequence
  • discovered by F Jacob and J Monod in 1961 (Nobel prize 1965)
17
Q

What does the Y gene encode for in lac operon?

A

= lactose permease, a membrane protein that transports lactose into bacterial cell

18
Q

What does the Z gene encode for?

A

= beta galactosidase, main purpose to cleave lactose into glucose and galactose
- low background level converts lactose to allolactose (the lac operon inducer)

19
Q

What does the lac I region encode for?

A

= lac repressor: protein that inactivates the lac operon in absence of the inducer allolactose
- repressor is a diffusable tetramer

20
Q

What does the lac O region encode for?

A

= the lac operator: a regulatory DNA sequence that works in cis (=lacO only affects regions downstream) to control lac operon, lac repressor can bind to the lac operator

21
Q

Where is the lac O region located?

A

= b/n lac promoter and lacZ gene

22
Q

How does the lac repressor protein work?

A

= binds to the lac operator (lacO) when there is no lactose (no allolactose inducer), this blocks the RNA polymerase from binding to the -35(binding) and -10(unwinding) sequences (promoter region) in the lac promoter b/c lac operator overlaps the initiation start
- means structural genes cannot be transcribed (lac Z, Y, and A genes) = OFF characteristic of lac operon

23
Q

How is the lac operon turned ON?

A
  • lac repressor binds inducer and is inactive

- lac operon is induced and genes Z, Y, and A are transcribed and translated

24
Q

What convention is used to denote the normal (wildtype) and mutant forms of the lac operon genes?

A

+ for wildtype and - for the mutant gene
so of I+P+O+Z+Y+A+ then allolactose is inducible for all lac operon enzymes
but if I+P+O+Z+Y-A- then allolactose is inducible only for lacZ

25
Q

What are constitutive lac mutants?

A

= mutations in the I gene (I-) or the operator sequence (O^C) that result in constitutive synthesis of downstream wild-type lac Z, Y, and A genes

  • OC mutations act only in cis on the downstream Z,Y, and A genes b/c mutation that prevents repressor from binding can only act downstream from operator
  • example: E coli partial diploid (=merozygote) has F’ (episome) lac sequences in addition to chromosomal lac genes
26
Q

What are lacI superrepressors?

A

= lacI^S

  • in these allolactose binding site in the lacI repressor gene is altered, lacI repressor is a superrepressor, it can never be inactivated by binding to the inducer allolactose (regardless of lactose being present)
  • Therefore, lacI repressor is always bound to the operator and prevents transcription of lac operon genes
  • LacIs is dominant over lacI+
27
Q

What are lacP-?

A

= mutations in the lac promoter that interfere w binding of RNA polymerase to promoter, thus preventing lac operon transcription from occuring
- mutations in -35 or -10

28
Q

So, is the lac operon always induced in the presence of allolactose?

A

= no, not if glucose is also present! b/c glucose preferred as carbon source and prevents lac operon induction ( = negative regulation by glucose is catabolite repression)

29
Q

How does glucose regulate the lac operon?

A

= indirectly through effects on cyclic AMP (cAMP levels)

  • cAMP is signal molecule
  • when glucose high: cAMP levels low
  • when glucose low/absent: cAMP high and cAMP binds another protein (catabolite activator protein, CAP), cAMP and CAP permit binding to a special region in lac promoter that promotes binding of RNA polymerase (to start transcription)
  • this is called positive control
30
Q

What is required for lac operon transcription?

A

= cAMP/CAP complex promoter binding

  • this helps to orient RNA polymerase and associated sigma factor to start transcription
  • therefore, cAMP/CAP exert positive control over lac operon transcription
31
Q

What is the only state in which lac operon is transcribed?

A
  • when the cAMP/CAP is present but there is no lac repressor (low glucose and lactose present and allolactose inducer formed)

MBG 2040 (9 decks)

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