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BIOL 142 Chapter Notes - Chapter 18.4-18.5: Arabinose, Regulatory Sequence, Operon

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bluemosquito338
30 Apr 2018
School
Emory University
Department
Biology
Course
BIOL 142
Professor
Patrick Cafferty

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18.4%Positive%Control%of%Transcription%
!!
In!positive!control,!an!activator!protein!binds!to!a!DNA!regulatory!sequence!
• When!bound!to!DNA,!the!activator!interacts!with!RNA!polymerase!to!increase!the!rate!of!initiating!
transcription!
• ara!operon:!a!set!of!three!genes!in!E.!coli!that!are!transcribed!into!a!single!mRNA!and!required!for!
metabolism!of!the!sugar!arabinose!
o Transcription!of!the!ara!operon!is!controlled!by!the!AraC!regulatory!protein!
o Arabinose!is!found!in!plant!cell!walls!
• Without!arabinose!in!the!environment,!the!ara!operon!is!not!transcribed!
• When!arabinose!is!present!in!the!environment,!the!transcription!of!ara!operon!is!turned!on!by!the!
AraC!protein!
o AraC:!the!regulatory!protein!in!the!ara%operon!
o araC:!the!regulatory!gene!in!the!ara!operon!
!!
Products!of!the%ara!operon!metabolize!arabinose!
!
• AraC!protein!works!in!positive!control!of!the!ara%operon!
• The!AraC!protein!is!allosterically!regulated!by!arabinose!
o When!bound!to!arabinose,!two!copies!of!the!AraC!protein!attach!to!an!initiator!that!lies!just!
upstream!of!the!ara%operon!promoter!
• Initiator:!regulatory!sequence!of!DNA!
o Once!AraC!is!bound!to!DNA,!it!can!also!bind!to!RNA!polymerase!
o This!interaction!between!AraC!and!RNA!polymerase!holoenzyme!helps!to!dock!the!
polymerase!of!the!promoter!and!accelerate!the!initiation!of!transcription!
!!
AraC!is!both!an!activator!and!a!repressor!
• In!the!absence!of!arabinose,!the!two!copies!of!the!AraC!protein!remain!together!
o One!remains!bound!to!the!inhibitor!
o One!now!binds!to!the!ara!operator!regulatory!site!
• In!this!configuration,!AraC!works!as!a!repressor!to!prevent!the!transcription!of!both!the!ara!
operon!and!the%araC!gene!
%%
18.5%Global%Gene%Regulation%
!!
Global!gene!regulation:!the!regulation!of!multiple!bacterial!genes!that!are!not!part!of!one!operon!
• How!bacterial!cells!manage!response!that!require!the!expression!of!hundreds!of!genes!
o To!compete!for!resources!bacteria!must!be!able!to!coordinate!the!expression!of!large!sets!of!
genes!
!!
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Document Summary

Arac protein: arac: the regulatory protein in the ara operon, arac: the regulatory gene in the ara operon. Products of the ara operon metabolize arabinose: arac protein works in positive control of the ara operon. The arac protein is allosterically regulated by arabinose: when bound to arabinose, two copies of the arac protein attach to an initiator that lies just upstream of the ara operon promoter. Arac is both an activator and a repressor. In the absence of arabinose, the two copies of the arac protein remain together: one remains bound to the inhibitor, one now binds to the ara operator regulatory site. In this configuration, arac works as a repressor to prevent the transcription of both the ara operon and the arac gene. Ex: red cross add drops of chlorine to drinking water in congo to treat the contaminated water so they don"t get cholera: the genes that make the bacterium a pathogen are turned on using a regulon.

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Related Questions

1) In the wildtype (the DNA sequence most commonly found in nature) lac operon, transcription is severely down-regulated. A mutation in lacI that causes an early stop codon in the sequence (called a lacI - strain) results in an operon that has high levels of transcription of the polycistronic m-RNA. What type of regulatory protein is the LacI protein (hint: we covered this type of protein in lecture)?

The Lac regulatory protein is called a repressor. (is this correct)

2) If a bacterium with a wildtype lac operon is grown in the presence of lactose, high levels of transcription result for a while, however, eventually transcription is again down-regulated. If, on the other hand, the sugar IPTG is present (instead of lactose), transcription levels are high and remain high. What function do IPTG and lactose share? How are they differently affected by the operon product? (need help with this question)

Lactose and IPTG affected by the operon are both inducers of the Lac-Operon. (is this correct)

3) What is “epigenetics”? What forms of gene regulation are usually involved in epigenetic control of a trait? Why do we have such a limited understanding of epigenetic processes? (need help with this question)

Epigenetics is the study of phenotype changes that do not change the DNA sequence. Genes can be turned on and off based on environment situations of past generations, (parents or even grandparents) were exposed. Examples of some environmental factors could be diet, stress, smoking, drinking alcohol, and diseases. Histone modification and methylation are usually involved in epigenetic control of a trait. (is this correct)

1.) What type of control best describes the lac operon when it is regulated by allolactose? Explain the features of this type of regulation using the lac operon as an example.

2.) What is positive gene regulation?

a) when an activator protein binds to the operator of a gene to turn expression off

b) when a repressor protein binds to the operator of a gene to turn expression off

c) when an activator protein binds to the operator of a gene to turn expression on

d) when a repressor protein binds to the operator of a gene to turn expression on

e) when a protein binds to DNA to change gene expression

3.) In the lactose operon of E. coli, what is the function of each of the following genes or sites: a) regulator, b) operator, c) promoter, d) structural gene Z and e) structural gene Y?

a) regulator gene

b) operator

c) promoter

d) structural gene Z

e) structural gene Y

4.) In a negative, repressible regulatory mechanism, transcription of the structural gene(s) occurs in

a) the presence of the co-repressor, but not in its absence.

b) the absence of the co-repressor, but not in its presence.

c) the presence of the inducer, but not in its absence.

d) the absence of the inducer but not in its presence.

e) the absence or presence of the co-repressor.

What are answers and can you explain them to me?

In the following points, I have summarized Lac operon in Ecoli. I need to check if this is on the right track? and where does 'cleavage of disaccharides' fit into this picture?

Lactose is a disaccharide and the bacteria need specific genes to manage catabolism

1) The lacl promotor gene synthesis the repressor protein which binds to the operator gene stopping the RNA polymerase from coding the lac operon genes

2)But when lactose is present, it needs the specific genes to catabolise lactose

3) So, When allolactose (a derivative of lactose)is present, it binds to the repressor protein, the repressor releases from the operator gene

4) This allows the RNA polymerase can attach to the DNA control region

5) And drive down the operon for transcription and induction of the enzyme lacZ(beta-galactosidase) lacY and lacA which breaks lactose down into glucose and galactose

6) When the lactose is all gone, the repressor returns to the operator in a feedback loop system