The repressor contains a lactose-binding site
The dominance of IS mutation is due to the inactivation of the allosteric site on the Lac repressor. In an IS/I− diploid cell, none of the lac structural genes are transcribed. The IS repressor lacks a functional allolactose-binding site (the allosteric site) and thus is not inactivated by an inducer. Therefore, even in the presence of an inducer, the IS repressor binds irreversibly to all operators in a cell, thereby blocking transcription of the lac operon.

ANIMATED ART: IS Lac superrepressor mutations