O control negativo implica a unión dun [[represor]] ao operador, que impide a transcrición.
* In ''negative inducible operons'', a regulatory repressor protein is normally bound to the operator, which prevents the transcription of the genes on the operon. If an [[inducer]] molecule is present, it binds to the repressor and changes its conformation so that it is unable to bind to the operator. This allows for expression of the operon.
* In ''negative repressible operons'', transcription of 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, certain molecules called corepressors are bound by the repressor protein, causing a conformational change to the active site. The activated repressor protein binds to the operator and prevents transcription.
Operons can also be positively controlled. With positive control, an [[activator protein]] stimulates transcription by binding to DNA (usually at a site other than the operator).
* In ''positive inducible operons'', activator proteins are normally unable to bind to the pertinent DNA. When an [[inducer]] is bound by the activator protein, it undergoes a change in conformation so that it can bind to the DNA and activate transcription.