Repressible Operon: 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.

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.

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.

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.

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.

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.

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.