lac Operon Model
Regulates production of enzyme B-galactosidase which is needed to break down lactose. It Consists of a cluster of 3 genes under control of one promoter and one operator. When lactose levels are low the LacI repressor protein binds to lac operator, covering part of the promoter region which blocks transcription of mRNA (does not allow RNA polymerase to bind). The Repressor protein binds to operator when lactose levels are low. When lactose levels are high, LacI repressor needs to be removed so lactose itself acts as an inducer or signal molecule. Lactose binds to LacI protein, changing its shape and it falls off operator allowing transcription of B-galactosidase to continue; High levels of lactose induce the Operon.
trp Operon Model
Regulates the production of the amino acid tryptophan. Consists of a cluster of 5 genes under control of one promoter. When tryptophan levels are high (diet), it acts as a corepressor and binds to trp repressor protein altering its shape so that it can bind to trp operator blocking transcription of mRNA. Repressor-tryptophan complex binds to operator when trp levels are high. When trp levels are low shape of tryptophan corepressor complex changes (due to loss of tryptophan) and falls off operator allowing transcription of tryptophan to continue; High levels of tryptophan represses the Operon.