Fig. 1.

Repression and expression ability of lac repressor–operator mutants. (A) Schematic representation of the E. coli lac system. β-Galactosidase (LacZ) and the lac permease (LacY) are coregulated by the repressor LacI. Expression is induced by IPTG. Red lines correspond to mutated positions. (B) The multimeric lac repressor in green bound to its operator DNA. Red indicates mutated positions responsible for specific repressor–operator binding. Y and Q are the mutated amino acid residues on positions 17 and 18 in the DNA binding helix of the lac repressor, and 4 and 4′ g–c and 5 and 5′ t–a are the mutated base pairs in the operator DNA. We note that the genotype represented here is YQ:tggt. (C) Characterization of 64 lac repressor–operator variants. The starting and final sequences are indicated by white and black circles, respectively. R is the inverse of the measured expression level in the absence of IPTG. E is the measured expression level in the presence of IPTG (Methods).