The present invention provides altered lac repressor proteins that recognize the lactose operator with increased affinity and have either normal or enhanced ligand responsivity. For example, the lac repressor Gln60Gly mutant protein exhibits increased binding affinity for lactose operator DNA, while maintaining near-normal responsivity to IPTG. Alternatively, the present invention provides modified repressors which exhibit responsiveness to an alternative ligand, such as arabinose, or have enhanced responsivity to IPTG. For example, Gln60Gly/Leu148Phe binds with wild-type affinity to lactose operator DNA and exhibits enhanced responsivity to IPTG. The present invention also provides for repressors that exhibit both characteristics: increased affinity for lactose operator and enhanced ligand responsivity. Enhanced ligand response enables induction of gene expression to be finely controlled by a researcher. DNA sequences encoding the altered lac repressor proteins and bacterial and eukaryotic cells containing altered lac repressor proteins are also provided.