FIG. 2.

Gene expression can be modulated by RExTET cassettes and can be taken over by PcL cassettes. β-Galactosidase activity measurements of RExTET and PcLlacZ fusions. Strains were grown up until stationary phase in M63B1glu medium at 37°C. Concentrations (nanograms per milliliter) of aTc, when added, are indicated below the bars. MG1655 grown in the presence of 1 mM IPTG was considered to demonstrate the wild-type expression of the lac operon. The experiments were performed in triplicate; error bars represent standard deviations of the means. All the results were qualitatively the same with cultures grown in LB medium (data not shown), with slight changes in the levels of enzymatic activity and aTc concentrations needed to reach the plateau. +, present; −, absent.