Synthesis mRNA of the operon depends on the repressor that blocks the transcription when bound by tryptophan that acts as a co-repressor.
The repressor interacts with inverted repeats at the major groove of the operator, thus allowing it to be fixed.
When there is few tryptophan, the repressor is inactive and does not attach to the operator. The RNA polymerase will bind to the promoter and transcribe the trp structural genes E, D, C, B, A (enzymes that make it possible to synthesize tryptophan).
Chorismate → Anthranilic acid → Phosphoribosyl-antranilate PRA Carboxyphenylamino-deoxyribulose phosphate CDRP → Indole-glycerol phosphate InGP → Tryptophan.