An operator is a segment of DNA that regulates the activity of the structural genes of an operon it is linked to, by interacting with a specific repressor or activator. It is a regulatory sequence for shutting a gene down or turning it "on".

In negative inducible operons, a regulatory repressor protein is bound to the operator and it prevents the transcription of the genes on the operon. If an inducer molecule is present, it binds to repressor and changes its conformation so that it is unable to bind to the operator. This allows for the transcription of the genes on the operator.

In negative repressible operons, transcription of the genes on the operon normally takes place. Repressor proteins are produced by a regulator gene but they are unable to bind to the operator in their normal conformation. However repressor molecules can bind to the repressor protein and change its conformation so that it can bind to the operator. The activated repressor proteins bind to the operator and prevent transcription.