Practice question

Super-repressor allele lacIs encodes LacI protein altered specifically in inducer binding pocket via substitutions that block allolactose or IPTG interaction yet preserve intact helix-turn-helix DNA binding domain and tetramerization interface. These mutant proteins remain tightly bound to operator even after inducer accumulation reaches high levels, rendering lac operon unresponsive to lactose signal, phenotype termed uninducible and non-inducible. Dominance over wild-type occurs in merodiploids because super-repressor occupies operator despite normal repressor induction, preventing transcription from either chromosome copy. This contrasts with lacI- constitutive deletion and wild-type inducible regulatory phenotypes distinguishing structural domain functions of LacI protein organization.