What is an operon and how does the lac operon work?

An operon is a cluster of genes under control of a single promoter, common in prokaryotes, that are transcribed together as a polycistronic mRNA. The lac operon in E. coli contains genes for lactose metabolism (lacZ, lacY, lacA) and is regulated by a repressor protein and CAP activator. In the absence of lactose, the repressor binds the operator blocking transcription. When lactose is present, it is converted to allolactose which binds and inactivates the repressor, allowing transcription. This represents negative inducible regulation.