Operons Continued
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Lilli Howard
BIOL/GEN 313
Dr. Rodermel/Dr. Tuggle
11/29/14
1. The mmm operon, which has sequences A, B, C and D encodes enzymes 1 and 2.
Mutations in sequences A, B, C, and D have the following effects.
Mmm Absent
Enzyme 1
Enzyme 2
+
+
+
+
+
+
-
No mutation
A
B
C
D
Mmm Present
Enzyme 1
Enzyme 2
+
+
-
a. Is the mmm operon inducible or repressible?
b. Indicate which sequence is part of the following components of the operon.
a. Regulator gene _____B_____
b. Promoter ______D______
c. Structural gene for enzyme 1 ____A____
d. Structural gene for enzyme 2 ____C____
2. A mutation at the operator site prevents the regulator protein from binding. What effect
will this mutation have in the following types of operons?
a. Regulator protein is a repressor in a repressible operon: operon never turned off
b. Regulator protein is a repressor in an inducible operon: constitutive transcription
3. For each of the following types of transcriptional control, indicate whether the protein
produced by the regulator gene will be synthesized initially as an active repressor,
inactive repressor, active activator, or inactive activator.
a. Negative repressible: inactive repressor
b. Positive repressible: active activator
c. Negative inducible: active repressor
d. Positive inducible: inactive activator
4. For each of the following, describe the production of B- galactosidase and permease with
a (+) or (-) for both lactose present and lactose absent conditions.
a. lacI+ lacP- lacO+ lacZ- lacY+ / lacI- lacP+ lacO+ lacZ+ lacYb. lacI+ lacP- lacOC lacZ+ lacY+
c. lacI+ lacP- lacOC lacZ- lacY+ / lacI- lacP+ lacO+ lacZ+ lacY-
a.
b.
c.
Lactose Absent
B-galactosidase Permease
-
Lactose Present
B-galactosidase Permease
+
+
-
1060 Hixson-Lied Student Success Center  515-294-6624  sistaff@iastate.edu  http://www.si.iastate.edu
5. LacI+ is (dominant/recessive) to LacI-. This is because LacI acts (cis/trans).
6. Explain why mutations in the lacO gene are cis in their effects.
7. Describe the three different types of mutations that are possible in structural genes.
1. gene product is present and inactive due to mutation, such as change of
one base for another that results in a different codon for a different
amino acid for another
2. the protein is not present (called a null mutation): e.g., this often occurs
when the gene or portions of the gene are deleted
3. different mutants in a gene usually display varying degrees of downregulation, from a small affect on protein activity to total abolition of
activity (null)
8. Describe catabolite repression and how it relates to the lac operon in E. coli.
Catabolite repression involves positive control in response to low glucose
Low glucose in the cell results in high levels of cAMP; cAMP binds to CAP (a DNA binding
protein, motif is HTH).
The cAMP-CAP complex binds just upstream of lacP and induces transcription along with the
presence of lactose.
1060 Hixson-Lied Student Success Center  515-294-6624  sistaff@iastate.edu  http://www.si.iastate.edu