FIG. 3.

Stability of the overproduction phenotype. (A) Changes in β-galactosidase activities in batch cultures of organisms bearing the cascade system in different configurations 6, 12, 24, 36, 48, and 72 h after the addition of salicylate. Activity is represented by using a logarithmic scale. A 1:200 dilution of each culture in fresh medium was made every 12 h except for the last 24 h. The bars indicate the averages for at least two independent experiments in which less than 25% error was observed. (B) Total-protein samples of CC1184S2PT97 and CC1184S2λpir(pTPm-lacZ) cultures taken at different times after induction, resolved by SDS-PAGE. An arrowhead indicates the band corresponding to β-galactosidase. (C) Stability of the overproduction phenotype of the system based on the T7 RNA polymerase: changes in β-galactosidase activities in batch cultures of E. coli NCM631(pIZ1207) grown and diluted as described above for panel A. Symbols: ▵, uninduced, no ampicillin; ▴, induced, no ampicillin; ○, uninduced, ampicillin; •, induced, ampicillin. For comparison, the β-galactosidase activities of induced cultures of CC1184S2PT97 (⋄) and CC1184S2λpir(pTPm-lacZ) (▪) without selective antibiotic pressure are also shown.