Physiological regulation of a decontrolled lac operon

The expression of the lac operon was studied under a variety of growth conditions in induced and in constitutive cells of Escherichia coli that carried different catabolite-insensitive lac promoters. Use of such "decontrolled" lac operons permitted a study of the expression of an operon that was presumably subject only to passive control. Since the use of toluenized cells was demonstrated not to be completely reliable, all enzyme assays were performed on sonic supernatant fluids. The cells contained different catabolite-insensitive promoters, which included the L1 and UV5 lac promoters, as well as others isolated in this study. There were three major observations. First, small but real carbon source effects were seen. Second, there was only a small change in beta-galactosidase specific activity with changes in the growth rate. This result implies a limited transcription and/or translation capacity within the cell. Third, at rapid growth rates, most promoters exhibited a decreased expression. The UV5 promoter, which was the "strongest" promoter, was an exception. A mechanism to explain this promoter-dependent control is discussed.