Model of Lac repression. (A) The lac operon. (B) 1. The lac promoter region. 2. Unrepressed transcription. 3. Repression by tetrameric Lac repressor binding to O1 and O3. The promoter site is unoccupied, however, current models have the CAP site always occupied. RNA polymerase cannot bind to the promoter. 4. Repression by tetrameric Lac repressor binding to O1 and O2. As mentioned above, the CAP site is probably always occupied. Here RNA polymerase can bind to the promoter, but cannot begin RNA synthesis because repressor blocks RNA polymerase.