text: The lacZYA operon houses genes encoding proteins needed for lactose breakdown. The lacI gene codes for a protein called "the repressor" or "the lac repressor", which functions to repressor of the lac operon. The gene lacI is situated immediately upstream of lacZYA but is transcribed from a lacI promoter. The lacI gene synthesizes LacI repressor protein. The LacI repressor protein represses lacZYA by binding to the operator sequence lacO.
  The lac repressor is constitutively expressed and usually bound to the operator region of the promoter, which interferes with the ability of RNA polymerase (RNAP) to begin transcription of the lac operon. In the presence of the inducer allolactose, the repressor changes conformation, reduces its DNA binding strength and dissociates from the operator DNA sequence in the promoter region of the lac operong. RNAP is then able to bind to the promoter and begin transcription of the lacZYA gene.