The lac operon in e. coli consists of genes that code for enzymes necessary for the breakdown of lactose. when lactose is absent, the operon is inactive because a repressor protein binds to a specific site in the lac operon. when lactose is present, lactose molecules bind to the repressor protein, causing the repressor protein to dissociate from the binding site. in the absence of glucose (a preferred energy source for bacteria), the protein cap binds to a regulatory site near the lac promoter to activate transcription of the lac operon.