6.12
Operone Reprimibile – Operone trp
L’operone trp in Escherichia coli rappresenta un classico esempio di operone reprimibile. Esso regola la sintesi del triptofano attraverso un controll…
L'operone E. coli trp contiene cinque geni strutturali per la biosintesi del triptofano, preceduti dal promotore, dall'operatore e dalla sequenza leader.
Il repressore trp , o TrpR, e gli attenuatori all'interno della sequenza leader, TrpL, regolano l'operone trp.
Quando abbondante, il triptofano lega il TrpR come corepressore. Il complesso triptofano-TrpR si lega all'operatore, bloccando la trascrizione.
Se l'inizio della trascrizione avviene nonostante la repressione, viene trascritto il TrpL, contenente quattro regioni attenuatrici che possono formare strutture di RNA stem-loop.
Il ribosoma traduce rapidamente i codoni tandem del triptofano dell'mRNA del peptide leader.
Di conseguenza, le regioni attenuatrici 3 e 4 dell'mRNA formano l'ansa del terminatore, terminando prematuramente la trascrizione a monte dei geni strutturali.
Quando i livelli di triptofano diminuiscono, il complesso triptofano-TrpR si dissocia, liberando l'operatore per la trascrizione e la traduzione in tandem.
Il ribosoma si blocca ai codoni tandem del triptofano in TrpL, permettendo alle regioni 2 e 3 di accoppiarsi per formare il ciclo anti-terminatore per la trascrizione continua.
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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.