Figure 2. Model of Lac repressor. (a) The lac system. (b) (1) The lac promoter region. (2) Unrepressed transcription. (3) Repression by tetrameric Lac repressor binding to O1 and O3. The catabolite repressor protein (CAP) site and the PI promoter are depicted as being unoccupied. There is no evidence for or against this. (4) Repression by tetrameric Lac repressor binding to O1 and O2. It is likely but not explicitly demonstrated that the CAP and RNA polymerase are bound to their sites. From Oehler S, Amouyal M, Kolkhof P, Wilcken-Bergmann Bv and Müller-Hill B (1994) Quality and position of the three lac operators of E. coli define efficiency of repression. EMBO Journal 13: 3348–3355.