Explain how a polycistronic structural gene is regulated by a common promoter and a combination of regulatory genes in a lac operon.

Lac operon is composed of three structural genes (z, y and a) which are involved in the metabolism of lactose. There is one promoter, one operator, and a regulatory (i) gene. The regulatory gene produces the repressor protein which binds to the operator site. This blocks the transcription by RNA polymerase. When lactose is present, the repressor protein binds to lactose and forms allolactose which cannot bind to the operator. The RNA polymerase binds to the promoter and the polycistronic mRNA is produced.