The operon is regulated by Lac repressor, the product of the lacI gene, which is transcribed from its own promoter (PI). The repressor inhibits transcription by binding to the lac operator (O).

They inserted these two sequences upstream of a reporter gene and used a colorimetric assay to determine ... create a DNA loop that was as strong as that could be seen in the lac repressor system,” ...

Mutations that disrupt the trpR gene lead to elevated production ... For example, repression of the lac operon by its repressor, called lacI, is inhibited by the ligand allolactose, to which ...