Some enzymes are termed inducible. This means that they are produced only in the presence of the substances they act upon. E. coli has three genes for the metabolism of lactose. These genes are clustered together along with their operator and promoter in an operon. In the absence of lactose a repressor molecule binds to the operator and blocks transcription of the 3 genes because RNA polymerase cannot attach to the DNA. When lactose is present it binds to the repressor and removes it. Now transcription can proceed. In this way, E. coli avoids producing the enzymes when it doesn’t need them.