In order for DNA to initiate the formation of RNA for subsequent translation, it must first unite with an RNA polymerase. The site for RNA polymerase attachment on the DNA is called the promoter. The operator lies within the promoter region (or overlaps it) so that attachment of the RNA polymerase is blocked when the repressor is present. In the presence of an inducer (lactate or a closely related isomer, in the case of the lac operon), the repressor is inactivated (and thus detaches from or fails to attach to the operator), and the operon turns on in the typical inducible fashion. RNA polymerase may then attach to the promoter site, and transcription can be carried out. Transcription occurs along the DNA template in a 3' to 5' direction. In the case of the lac operon, for example, a singleRNAtranscript is synthesized for the three structural genes of the lac operon. Before translation, processing of this single RNA molecule will occur to permit the generation of three separate enzymes.
