operator gene

A site on the DNA adjacent to a *structural gene that acts as part of a molecular 'switch' and determines whether or not transcription of the structural gene will occur. The switch may be turned 'off' by a *repressor molecule binding to the operator, so preventing mRNA polymerase reaching the DNA. Conversely, absence of a repressor allows mRNA polymerase to bind to the DNA, i.e. the switch-is 'on'. Other methods by which the operator genes function are also known. See operon.