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BIOL 221
Microbiology
The lac operon
KEY
2008
Name: ____________________________
1. The “box” below represents the lac operon. Indicate with boxes the relative positions of all the regulatory
regions and the three genes. Label the regulatory regions and use genes 1 – 3 for the coding genes.
CAP binding site Promoter Operator
Gene1
Gene2
Gene3
CAP
2. What role does –galactosidase play in lactose metabolism?
It breaks down lactose (a disaccharide) into galactose and glucose (monosaccharides).
3. What is the inducer of the lac operon? __Lactose or allolactose__ (note: cAMP is an activator)
4. What role does the repressor (product of the lacI gene) play in control of transcription of the lac operon?
It is inhibits transcription when physically bound to the regulatory region.
5. What effect does the inducer have on the lacI gene product?
It has an allosteric effect on the repressor, changing its shape so it can no longer bind to DNA
(the operator site).
6. What is the region of the lac operon where the repressor protein binds to the DNA called? _operator_
7. Where is this in relation to the promoter region of the lac operon? ___downstream___
8. Why is transcription blocked when the lacI inhibitor is bound to DNA?
It physically prevents the RNA polymerase from moving downstream and transcribing the
coding (structural) genes of the lac operon.
9. Why is this type of gene regulation called “negative”?
Transcription is “turned off” by the presence of a repressor.
10. What is the activator protein called that positively regulates the lac operon? Spell it out.
Catabolite activator protein or cAMP receptor protein
11. Fill in the following table.
Growth Medium
presence of glucose,
no lactose
presence of glucose and
lactose
presence of lactose,
no glucose
12.
lacI bound to DNA?
(Y or N)
Y
CAP bound to DNA?
(Y or N)
N
lac operon turned
on? (Y or N)
N
N
N
N
Y
N leaking
expression
Y effective
–galactosidase enzyme in the constitutive mutants different from normal
E. coli cells?
The enzyme is expressed all the time, regardless of the environment.
13. What genetic experiment did Jacob & Monod perform to demonstrate that the constitutive mutants were
lacI-, i.e. defective in the lacI gene? (Think about the transfer of DNA).
Conjugation of a bacterium containing an F’ plasmid that has a normal lacI gene with the
constitutive mutant (female) restored normal regulation of the lac operon in the mutant.