FIG 6.

Transcriptional activity of the ect wild-type promoter and two of its mutant derivatives in response to sustained osmotic stress. (A to C) E. coli strains (MC4100) harboring either the wild-type ectB-lacZ reporter fusion plasmid pGJK4 (A), a plasmid (pPH8) carrying a point mutation (Mut 12) (Fig. 4A) in the ect −35 region (B), or a plasmid (pPH11) carrying a mutant ect promoter (Mut 15) (Fig. 4A) that was changed in its −10 region, −35 region, and spacer length to the consensus sequence of Sig⁷⁰-type E. coli promoters (78) (C) were grown at various salinities. (D) Cells of strain MC4100 or FF4169 (otsA1::Tn10) carrying the wild-type plasmid pGJK4, pPH8, or pPH11 were grown in MMA with 0.3 M NaCl. Cultures were harvested and processed for β-galactosidase enzyme activity when they reached an optical density (OD₅₇₈) of about 1.8. The data shown were derived from four independently grown cultures, and each enzyme assay was performed at least twice. β-Galactosidase enzyme activity is given in Miller units (MU) (108).