TrpOperon

The trp operon is a classic example of a repressible operon in bacteria that controls the biosynthesis of the amino acid tryptophan. In Escherichia coli and many related species, the operon contains five structural genes: trpE, trpD, trpC, trpB, and trpA, which encode the enzymes of the tryptophan biosynthesis pathway. The genes are co-transcribed from a single promoter into a polycistronic mRNA. Upstream there is a regulatory leader sequence, trpL, which contains a short open reading frame with two tryptophan codons and functions in transcription attenuation.

Regulation involves two complementary mechanisms. The TrpR repressor, encoded in a separate trpR gene, binds tryptophan as a corepressor. When tryptophan is plentiful, the TrpR-tryptophan complex binds to the operator site overlapping the promoter and prevents transcription of the operon. When tryptophan is scarce, the repressor is inactive, allowing transcription to proceed.

In addition to repression, the operon is regulated by attenuation at the trpL leader sequence. The leader contains a ribosome binding site and a short ORF with two Trp codons. Translation of this leader affects the formation of alternative mRNA structures. High tryptophan levels lead to termination of transcription via a terminator hairpin, whereas low tryptophan causes ribosome stalling at the Trp codons, allowing transcription to proceed into the structural genes.

The trp operon is a paradigmatic model for gene regulation, illustrating feedback inhibition, repression, and attenuation in prokaryotes.