The lac I gene is the regulatory gene of the lac operon. The lac I gene encodes the lac repressor protein. When lac repressor protein is bound to the lac operator DNA sequence, mRNA synthesis from the structural genes of the operon is blocked. In a lac I mutant, active repressor protein is not synthesized. As a result, mRNA synthesis from the lac operon is constitutive, and there is constitutive expression of the lac Z gene product, ß-galactosidase.
The structure of the complex formed between the lac repressor protein DNA binding domain and the lac operator DNA sequence is shown in the figure: