Unformatted text preview: otype I-O+Z+/I+ Lactose No lactose + Conclusion? - “I” is a diffusible factor I+OCZ+/O+ + + Operator must function
in cis ISO+Z+/I+ - - IS is dominant Positive control and catabolite repression • E. coli and other bacteria metabolize glucose preferentially in the
presence of lactose and other sugars. • Why?
• Glucose requires less energy to metabolize than do other
sugars because it enters glycolysis without modiﬁcation. Positive control and catabolite repression • When glucose is available:
• genes that participate in the metabolism of other sugars are
repressed (catabolite repression). • Catabolite: a substance produced by catabolism, which is the
set of metabolic pathways that break down molecules into
smaller units and release energy. Positive control and catabolite repression • How is expression of the lac operon repressed when glucose is
present? • Low levels of glucose within a cell stimulate high levels of a
modiﬁed nucleotide called adenosine-3’, 5’-cyclic
monophosphate, or cAMP (pronounced cyclic AMP) • cAMP binds to a protein called Catabolite Activator Protein,
or CAP • CAP is an activator that increases the binding of RNA
polymerase to many promoters, including the lacP. Catabolite Activator Protein (CAP) Binding of the cAMP-CAP complex
Binding of the cAMP-CAP
complex to DNA produces a
sharp bend in the DNA that
activates transcription So...
• Why is this called “catabolite repression”?
• Catabolites of glucose repress adenylate cyclase (AC)
• No AC = no cAMP
• Thus, when glucose catabolites are present, CAP is not active
• Thus, the lac operon is repressed when glucose levels are high For Monday: Finish Chapter 16
Chapter 17 (Control of Eukaryotic Gene Expression)...
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