Operons are a group of closely linked genes that produce a single messenger RNA molecule in transcription and consist of structural genes and regulating elements such as an operator and promoter. Types of Operons ? Inducible operons such as lactose operon must be activated by inducers, e.g., lactose operon regulates lactose catabolism. ? Repressible operons Repressible operon is ordinarily active and produces the protein they are meant to make unless the protein is present to activate the repressor. An example is tryptophan operons which regulate tryptophan synthesis.