Figure 1. Schematic representation of the lac pathway.

The lac operon is composed of three genes controlled by the same promoter: lac Z, lac Y, and lac A. The lac pathway also involves the constitutively expressed repressor LacI. It represses the transcription from the lac promoter by attaching to the operator sequence lacO1. Inducers, such as IPTG, deactivate the repressor LacI and thus stimulate the synthesis of the gene products LacZ, LacY, and LacA. The rate of production of the three lac proteins is denoted by and it depends on the sequence of lac O1, on the presence of inducer inside the cell, and on the growth rate. All three lac genes are transcribed with the same rate, hence LacZ can be used as a reporter for the whole operon. LacY transports molecules such as IPTG inside the cell with a rate , which depends on the concentration of these molecules. One proton is transported with each substrate molecule [7]. Growth (measured by the Malthusian fitness ) dilutes the internal molecules, thus lowering their concentrations. The strains used in this study differ by the lacO1 sequence and are grown in various IPTG concentrations.