The operon is regulated by Lac repressor, the product of the lacI gene, which is transcribed from its own promoter (PI). The repressor inhibits transcription by binding to the lac operator (O).

The genes and important sequences for the regulation of the lac operon are organized such that several mediators can fine tune expression of the three genes in the operon itself—lacZ, lacY, and ...

The answer lies in regulating the operon. As trp levels increase, trp binds to trpR, causing a conformational change that allows binding to the operator and repression of gene expression.