Tryptophan absent, repressor inactive, operon on.
In the repressible trp operon, 5 genes encoding the enzymes to synthesize tryptophan are regulated by a promoter and an operator.

When tryptophan is absent, the repressor (with its own promoter) is inactive and the operon is on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon's genes.