Schematic of the OROlac engineered promoter. The promoter fuses three operator sites, one of which (Olac) yields repression when the LacI tetramer is bound to it, while binding of the CI dimer to OR2 yields approximately ten-fold activation (the OR1 site assists activation by cooperatively enhancing CI binding to OR2 when CI is bound to OR1). In the experimental system, CI and LacI are produced in the cell by external promoters, not shown here. The Green Fluorescent Protein (GFP) product is then used to monitor the output of the promoter by flow cytometry.
Adalsteinsson et al. BMC Bioinformatics 2004 5:24 doi:10.1186/1471-2105-5-24