Top: The operator is shown in red, part of the DNA. It is bound to the repressor (green), which prevents RNA polymerase (yellow) from transcribing the target genes (6,7,8).
Bottom: The green repressor is inhibited by the white molecule. RNA polymerase (yellow) is free to transcribe the target genes (6,7,8). This particular process shows the operator's role in the lac operon. Lactose is the white molecule in the bottom diagram.
In genetics, an operator is a segment of DNA to which a transcription factor binds to regulate gene expression by repressing it. The protein that does this is called a repressor. Repressors bind to operators to prevent transcription.
The main operator (O2) in the classically defined lac operon is located slightly downstream of the promoter. Two additional operators, O1 and O3 are located at -82 and +412, respectively.
The repressor protein physically obstructs the RNA polymerase from transcribing the genes.
An inducer (small molecule) can displace a repressor (protein) from the operator site (DNA), resulting in an uninhibited operon.
Alternatively, a corepressor can bind to the repressor to allow its binding to the operator site. A good example of this type of regulation is seen for the trp operon.