6.11
Induzierbare Operons – lac-Operon
Das lac-Operon von Escherichia coli dient als Modell zum Verständnis der induzierbaren Genregulation und metabolischen Flexibilität. Es integriert ein…
Der Laktosestoffwechsel von E. coli wird durch die Strukturgene des Lac-Operons gesteuert, die lokal durch den laktoseabhängigen LacI-Repressor reguliert werden.
Darüber hinaus reguliert der Katabolit-Repressionsmechanismus diese Gene auf der Grundlage der Glukoseverfügbarkeit über ein globales regulatorisches Protein, das cAMP-Rezeptorprotein oder CRP.
Wenn Glukose im Überfluss vorhanden ist, wird die zyklische AMP- oder cAMP-Bildung gehemmt.
Kein CRP-cAMP-Komplex kann sich ohne cAMP bilden, so dass die CRP-Bindungsstelle stromaufwärts des lac-Operons ungebunden bleibt. Gleichzeitig bleibt der LacI an den Bediener gebunden.
Die Hemmung der lac-Operon-Expression führt dazu, dass E. coli zunächst nur Glukose metabolisiert, selbst wenn Laktose verfügbar ist, wodurch die Zellenergie erhalten bleibt.
Wenn der Glukosespiegel sinkt, steigt der cAMP-Spiegel. Es wird der cAMP-CRP-Komplex gebildet, der die DNA bindet und die Promotorrekrutierung der RNA-Polymerase verbessert.
Eine kleine Menge verfügbarer Laktose erzeugt Allolactose, die als Induktor fungiert, der den LacI-Repressor bindet.
Die Allolactose-Bindung löst eine allosterische Veränderung aus, die LacI vom Operator freisetzt und die Transkription des lac-Operons ermöglicht.
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Q1: What are the three structural genes in the lac operon and what do they encode?
The lac operon contains three structural genes: lacZ encodes β-galactosidase, which breaks down lactose; lacY encodes lactose permease, which transports lactose into the cell; and lacA encodes thiogalactoside transacetylase, which modifies lactose metabolites. These genes are transcribed together as a single mRNA under environmental control.
Q2: How does allolactose trigger derepression of the lac operon?
When lactose is present, a small amount converts to allolactose, which acts as an inducer. Allolactose binds to the LacI repressor protein, causing an allosteric change that detaches the repressor from the operator. This derepression allows RNA polymerase to access the promoter and initiate transcription of the structural genes.
Q3: Why does E. coli preferentially metabolize glucose over lactose?
When glucose is abundant, cyclic AMP (cAMP) formation is inhibited, preventing the cAMP-CRP complex from forming. Without this complex binding to the CRP-binding site, RNA polymerase recruitment to the lac promoter is inefficient. This catabolite repression mechanism ensures E. coli conserves energy by using glucose first, even when lactose is available.
Q4: What role does the cAMP-CRP complex play in lac operon regulation?
When glucose levels drop, cAMP levels increase and form a cAMP-CRP complex. This complex binds to the CRP-binding site upstream of the lac promoter, enhancing RNA polymerase recruitment and significantly increasing lac operon transcription. The cAMP-CRP complex acts as a global regulatory signal for metabolic flexibility.
Q5: How do local and global regulation work together in the lac operon?
Local regulation by the LacI repressor responds to lactose presence through allolactose binding, while global regulation via the cAMP-CRP complex responds to glucose availability. Both mechanisms must permit transcription for full lac operon activation. This dual control ensures E. coli efficiently manages metabolic resources by prioritizing glucose utilization and switching to lactose metabolism only when glucose is depleted.
Q6: What happens to the lac operon when both glucose and lactose are present?
When both glucose and lactose are present, the LacI repressor remains bound to the operator because cAMP levels are low and the cAMP-CRP complex cannot form. Even though lactose is available, the lac operon remains repressed. E. coli prioritizes glucose metabolism, conserving energy by avoiding unnecessary lactose enzyme production.
Q7: How does the lac operon demonstrate metabolic efficiency in changing environments?
The lac operon integrates local lactose sensing with global glucose monitoring, allowing E. coli to adapt its metabolism to available nutrients. When glucose is abundant, the system suppresses lactose genes; when glucose depletes, cAMP rises and allolactose derepresses the operon simultaneously, enabling rapid lactose utilization. This ensures survival and adaptability in fluctuating nutrient conditions.