BIOLOGY 207 - Dr. McDermid
Lecture #10
Further regulation of the lac operon.
Readings for lectures #4 and #5:
Griffiths et al, 7th Edition:
Ch. 11 pp 335-344
Ch 7 pp210-219 (skim only, beginning and end)
Problems for lectures #4 and #5:
Griffiths et al, 7th Edition: Ch. 11 Tier 1: #1,2,3,4,8 Tier 2: #9,14,15
Concepts:
How is the lac operon regulated? (continued)
1. The Oc mutations in the lac operon define a cis-acting locus
2. The lac operon is also under positive control and is affected by the level of glucose.
trans-acting locus
cis-acting locus
- a genetic region affecting the activity of genes on that same DNA molecule
Operator mutants OC mutants
Figure 11-8, Table 11-1
Jacob and Monod proposed the
"operator element" in the lac operon.
How were these mutants obtained?
This led to the recovery of OC - operator constitutive mutants.
O C mutants - constitutively expressed
- no inducer -
Lecture #5
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- with inducer -
Genetic mapping positioned the OC mutants to between I locus and Z locus.
O C is dominant in the cis position
cis dominance - the ability of locus to influence the expression of one or more adjacent loci on the
same chromosome, as occurs in lac operator mutants of E.coli
Summary of Negative control
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Repressor protein -
Inducer -
Positive control
Catabolite Repression of the lac Operon:
E.coli has the adaptive response that it will utilize glucose before lactose
Unknown Catabolic breakdown product of glucose acts to prevent expression of the lac operon
even if lactose is present.
Glucose levels affect this system
Figure 11-11
1- High glucose level
2- Low glucose level
Requires cAMP to form a complex with a protein called CAP (crp gene)
Mechanism
CAP + cAMP form a complex ‡ activate the expression of the lac operon when bound to the
CAP binding site
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Mutations in crp gene lead to a non-functional CAP protein ‡ no lac expression in either the
presence or absence of the inducer.
Summarized in Figure 11-12
3 situations
1) + glucose - lactose
‡
2) + glucose + lactose ‡
3) - glucose + lactose ‡
- CAP-cAMP system is positive control
Result:
Lac transcription occurs only when needed and useful
Comparison of Positive and Negative Control
Figure 11-13
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Lecture notes: Copyright © 2002 Heather McDermid and the Department of Biological Sciences, University of Alberta
Images are Copyright©2000 by W.H. Freeman & Co. in Griffiths et al, Introduction to Genetic Analysis
Lecture #5
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