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.

Shelby is an assistant editor for The Scientist. She earned her PhD from West Virginia University in immunology and microbiology and completed an AAAS Mass Media fellowship.

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).