Lecture.18.Fall.2013

Why glucose requires less energy to metabolize than

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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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