6.12
Reprimierbares Operon – Trp-Operon
Das trp-Operon bei Escherichia coli ist ein Beispiel für ein reprimierbares Operon. Es reguliert die Synthese von Tryptophan durch eine repressorvermi…
Das E. coli trp Operon enthält fünf Strukturgene für die Tryptophan-Biosynthese, denen die Promotor-, Operator- und Leader-Sequenz vorangestellt ist.
Der trp Repressor oder TrpR und die Abschwächer innerhalb der Leader-Sequenz, TrpL, regulieren das trp-Operon.
Wenn Tryptophan reichlich vorhanden ist, bindet es TrpR als Corepressor. Der Tryptophan-TrpR-Komplex bindet an den Bediener und blockiert die Transkription.
Erfolgt die Initiierung der Transkription trotz Repression, wird das TrpL transkribiert, das vier Dämpfungsregionen enthält, die Stammschleifen-RNA-Strukturen bilden können.
Das Ribosom übersetzt schnell die Tandem-Tryptophan-Codons der Leader-Peptid-mRNA.
Infolgedessen bilden die Attenuatorregionen 3 und 4 in der mRNA die Terminatorschleife, die die Transkription stromaufwärts der Strukturgene vorzeitig beendet.
Wenn der Tryptophanspiegel sinkt, dissoziiert der Tryptophan-TrpR-Komplex und gibt den Operator für die Tandemtranskription und -translation frei.
Das Ribosom bleibt an den Tandem-Tryptophan-Codons in TrpL stehen, so dass sich die Regionen 2 und 3 paaren können, um die Anti-Terminator-Schleife für die fortgesetzte Transkription zu bilden.
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Q1: What are the structural components of the trp operon?
The trp operon contains five structural genes (trpE, trpD, trpC, trpB, and trpA) that encode enzymes for tryptophan biosynthesis. These genes are preceded by a promoter, operator, and leader sequence (trpL). The leader sequence is critical for attenuation, a regulatory mechanism unique to prokaryotes that provides fine-tuned control over gene expression.
Q2: How does tryptophan act as a corepressor in the trp operon?
When intracellular tryptophan levels are high, tryptophan binds to the TrpR repressor protein, acting as a corepressor. This binding induces an allosteric change that enables TrpR to bind the operator, physically blocking RNA polymerase from transcribing the structural genes and shutting down tryptophan biosynthesis.
Q3: What happens during attenuation when tryptophan levels are high?
When tryptophan is abundant, the ribosome rapidly translates the leader peptide, including its tandem tryptophan codons. This allows attenuator regions 3 and 4 in the mRNA to form a terminator loop, prematurely halting transcription upstream of the structural genes and preventing unnecessary tryptophan synthesis.
Q4: Why does the ribosome stall at tryptophan codons when tryptophan is scarce?
When tryptophan levels are low, insufficient tryptophan-charged tRNAs are available. The ribosome stalls at the tandem tryptophan codons in the leader sequence because it cannot find the required tRNA to continue translation, triggering an alternative regulatory pathway that permits transcription.
Q5: What is the anti-terminator loop and how does it permit transcription?
The anti-terminator loop forms when regions 2 and 3 of the leader sequence pair together. This structure prevents the formation of the terminator loop, allowing RNA polymerase to continue transcribing the structural genes and enabling tryptophan biosynthesis when tryptophan-charged tRNAs are scarce.
Q6: How does the trp operon demonstrate dual regulatory control?
The trp operon uses two complementary mechanisms: repression by TrpR blocks transcription initiation when tryptophan is abundant, while attenuation fine-tunes expression based on real-time availability of tryptophan-charged tRNAs. This dual system ensures tryptophan synthesis is tightly coupled to cellular needs through coordination of gene expression processes in bacteria.
Q7: What role do stem-loop structures play in trp operon regulation?
The leader sequence contains four attenuator regions capable of forming secondary RNA structures called stem-loops. Depending on ribosome position and tryptophan availability, these regions pair to form either a terminator loop that halts transcription or an anti-terminator loop that permits transcription, enabling responsive regulation.