FIG. 2.
Semilogarithmic plot of β-Gal activities in the presence of inducer versus β-Gal activities in the absence of TC. β-Gal expression was determined as described by Miller (15) with cells grown at 37°C in Luria-Bertani medium supplemented with the appropriate antibiotics. For measuring inducibility, TC was added at a concentration of 0.2 μg/ml. Three to four independent cultures were assayed for each strain, and measurements were carried out at least twice. The open circles correspond to the TetRs mutant TetR Δ164–166 (0.8% ± 0.0% β-Gal activity under repressed conditions and 0.9% ± 0.0% activity under induced conditions) and the TetR Δ164–166 double mutants with substitutions within the HTH. The mutations and the corresponding activities under repressed and induced conditions, respectively, are as follows: TG40, 1.6% ± 0.1% and 31% ± 1.6%; TQ27, 1.9% ± 0.1% and 42% ± 3.0%; VS36, 2.0% ± 0.1% and 31% ± 2.2%; LT41, 9.6% ± 0.5% and 68% ± 2.7%; QG38, 10% ± 0.9% and 74% ± 3.0%; VW36, 15% ± 0.7% and 85% ± 2.5%; WG43, 17% ± 2.1% and 87% ± 4.4%; KT48, 20% ± 1.1% and 82% ± 2.9%. The filled diamonds represent the suppressors isolated or constructed for the mutant TetR Δ164–166. The reference line was fitted by linear regression. Mutants above or on this line are labelled.