Biology 2581B Lecture Notes - Lecture 15: Stem-Loop, Trp Operon, Tryptophan

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18 Apr 2016

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A model of induction for the lac operon: This hypothetical model gave rise to several testable predictions. 1) the repressor protein must physically bind the operator sequence; this was determined by dna footprinting experiments establishing the region of dna to which a protein binds. 2) the repressor protein must physically bind the wild-type operator sequence, but not the lacoc mutant; this was determined by attaching a radioactive tag to the lac repressor protein so it can be followed in the experiment. 3) the repressor protein must have two distinct domains, one that binds dna and one that binds the inducer. Laci - mutations that affect dna binding all cluster in the n-terminus of the protein, while laci s mutations affecting inducer binding all cluster in the main body of the protein. The laci repressor is a tetramer of four identical laci encoded subunits that binds dna through the helix-turn-helix motif of the dna- binding domain.

Related Questions

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?

1.)A.)The lac operon is inducible by the presence of lactose. Loss-of-function mutations in the lacI gene, which encodes the lac repressor (pick all that apply):

A.)slow down the isomerization of lactose into allolactose
B.)decrease the amount of permease activity in the cell in the presence of lactose.
C.)prevent loss-of-function lacZ mutations from making the lac operon uninducible
D.)are recessive to wild type

B.)Mutations that eliminate the lacO operator site (pick all that apply):

A.)slow down the isomerization of lactose into allolactose
B.)decrease the amount of beta-galactosidase in the cell in the presence of lactose
C.)prevent lacS superrepressor mutations from making the lac operon uninducible
D.)are recessive to wild type

C.)Loss-of-function mutations in the lacZ gene, which encodes beta-galactosidase (pick all that apply):

A.)slow down the isomerization of lactose into allolactose
B.)decrease the amount of permease activity in the cell in the presence of lactose
C.)prevent lac repressor from binding the lacO operator site
D.)prevent IPTG from inducing the lac operon

D.)You are studying the Tory operon, which encodes the TorA and TorB genes, and is normally inducible by Torose. You find a mutation, m1, that causes the Tory operon to be uninducible. You examine the merodiploid (m1 TorA+ TorB-)/(+ ToryA- TorB+) and find that Tor A is uninducible and TorB is inducible. You conclude that:

A.)m1 acts in cis
B.)m1 acts in trans

ivorydog394

a) If a lacI - bacterium is grown in the presence of the sugar X-gal, a blue dye appears; however, the concentration of X-gal decreases over time. What has happened to the X-gal?

b) If a wildtype bacterium is grown in the presence of the sugar X-gal, transcription of the polycistronic m-RNA does not occur. What is X-gal unable to do? Without adding lactose (or genetically modifying the bacteria), how could you fix this problem (can you fix this problem by adding IPTG)

Can you explain the following answer in simple terms? Still do not understand what happens to the X-gal, and what x-gal is unable to do. Thank you

Answer A)

X-gal stands for 5-bromo-4-chloro-3-indolyl-Î²-D-galactopyranoside which has glycosidic bond between galactose and 5-bromo-4-chloro-3-hydroxyindole. It gets cleaved by beta-galactosidase translated from lac operon.

If lacI- mutant is provided with X-gal, because of mutant lacI gene, no repressor will be formed and hence the operon is swiched on.

The enzyme beta-galactosidase cleaves X-gal which results in decreased concentration of X-gal with time.

X-gal gets cleaved to form 5-bromo-4-chloro-3-hydroxyindole which then dimerizes to derivative of indigo and hence blue color appears.

Answer b

X-gal cannot form allolactose or its analogue (inducer) and so in wild type where lacI gene is normal and forms repressor. So, in absence of allolactose, repressor inducer complex is not formed and so, operon remains switched off and hence no beta-galactosidase is formed.

This problem can be solved by adding IPTG i.e. Isopropyl beta-1-thiogalactopyranoside which mimics the action of allolactose and so can switch on the operon.

Biology 2581B Lecture Notes - Lecture 15: Stem-Loop, Trp Operon, Tryptophan

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