Microbiology 3-4

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Microbiology 3-4
2013-11-12 19:48:12

Study guide 3-4
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  1. Describe the negative transcriptional control of the trp operon of E.coli
    • 1. Operons only function when tryptophan is NOT present
    • 2. TRP repressor is synthed in an inactive form-it cannot bind to trp operator when tryptophan levels are low.
    • 3. When Tryptophan levels go up it acts as a corepressor binding to the repressor and activating it, which prevents transcription
  2. Describe negative and positive transcriptional control of the ARA operon in E.coli
    1-Arabinose not preseent (repressor).(Negative control) AraC binds araI and another binds araO2. These two bend DNA, preventing RNA poly from binding to the promoter of the ARA operon, stoping transcription. AraC is the repressor here.

    2-Arabinose is present (activator). (Positive control)Arabinose binds to AraC and prevents the molecules from interacting, breaking the DNA loop. the binding of the two AraC complexes to the araI site promotes transcription. So when Arabinose is present AraC is an activator.
  3. Explain how two component signal transduction systems work:
    These are systems that react to stuff that doesn’t make molecules (temperature, O2 levels, osmolarity).

    • Sensor Kinase-spans the plasma membrane, can sense specific changes
    • and communicate xross the membrane.

    Response Regulator protein-binds to DNA when activated by a sensor kinase. Acts as a repressor or activator. It can tell the cell to transcribe stuff it needs or inhibit and say we don’t need it

    • Phosphorelay systems- when more proteins participate in the transfer
    • of phosphoryl groups. Longer pathways.
  4. Describe the regulation of the production of porins in E.Coli
    • OmpF and OmpC porin proteins.  High osmotic pressure= OmpC, low is OmpF. In
    • the intestinal tract you have OmpC.

    • Sensor Kinase- called the  EnvZ. It is
    • integral membrane. OmpR response regulator protein that regulates OmpC and
    • OmpF. OmpR binds to DNA, at low osmolarity EnvZ is inactive, it transfers a
    • phosphoryl group to OmpR which regulates porin transcription, repressing OmpF and activating OmpC.

    • OmpC doesn’t let water pass through
    • easily. OmpF does let water pass through. OmpR gets the phosphoryl group and it
    • will produce less OmpF, but will increase OmpC gene

    OMPR is a repressor and activator.
  5. Describe Attenuation:
    Terminating transcription within the leader region. This is a second point where you can terminate transcription (either before it starts or just after it starts)
  6. Describe the process of attenuation in the regulation of transcription elongation involving the trp operon
    • 2 and 3 join in low Tryptophan
    • concentrations, transcription continues.  Speed of the translation determines if it will continue (speed is affected by presence of tryptophan)

    • 3 and 4 join to stop transcription if
    • Tryptophan is high. 2 and 3 join in low tryptophane continuing transcription.

    • 1 and 2 and 3 and 4 forming means NO
    • tryptophan.
  7. What is a riboswitch?
    the leader region that turns transcription on or off.
  8. Describe the mechanism by which riboswitches control transcription elongation involving the rib operon of Bacillus subtilis
    • When transcription of RIB begins sequences in the leader region fold
    • into the RNF element, which binds FMN and alters the folding of the leader
    • region, making a terminator (transcription stop).