EXAMPLE FOR POSITIVE CONTROL - High glucose levels low...

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EXAMPLE FOR POSITIVE CONTROL The lac operon is also subject to positive regulation . But first some negative regulatory appearances : If E. coli has a choice, it will always prefer glucose over any other sugar . For example, when E. coli is grown in a medium with both glucose and lactose present, it cannot metabolize lactose, because glucose somehow prevents the transcription from the lac operon even in the presence of the inducer (lactose). However, glucose does not act via a repressor on the lac operon but acts via the inability of an activator to perform its function . Glucose levels are correlated to the concentration of cAMP via the enzyme that synthesizes cAMP, adenylyl cyclase.
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Unformatted text preview: High glucose levels ------ low activity of adenylyl cyclase ------ low cAMP levels Low glucose levels ----- high activity of adenylyl cyclase ------ high cAMP levels only at high cAMP levels and therefore at low glucose concentrations can cAMP bind to a trans-acting factor called catabolite activator protein ( CAP ). This binding makes it possible for CAP to bind to its specific regulatory sequences on the DNA (Fig. 6.10). Many operons that are important for the metabolism of other carbon sources than glucose contain such a specific regulatory DNA sequence for CAP-binding. In the lac operon this sequence lies just upstream of the promoter....
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