The Lac promoter initiates transcription of the genes of the Lac
It is regulated by the actions of the Lac repressor binding to the
Lac operator in the absence of lactose therefore preventing RNA
polymerase from binding to Lac promoter.
The repressor is inactivated in the presence of lactose, which
binds to the repressor. This causes a confomational change that
prevents it from binding to the operator region therefore enabling
the transcription machinery to bind to the promoter leading to the
expression of the three Lac genes of the operon.