The Lac Operon and Its Role in the Breakdown of Lactose in Escherichia Coli
An operon is a region of DNA that controls the transcription of several genes by activation or regulation of a promoter. Operons are only found in prokaryotic cells, more specifically in Escherichia Coli. The Lac operon is used to break down lactose into glucose and fructose within the cell by breaking the beta 1,4-glycosidic linkage between lactose. It has three genes under its control: Lac Y, Lac Z, and Lac A. The Lac Z gene encodes for the enzyme beta glalactosidase which is responsible for cleaving lactose into fructose and glucose for use within the cell. The Lac Y gene is responsible for the production of the protein pump lactose permease which is a symporter used to bring glucose along with protons into the cell. Symporters are types of pumps used to pump two types of solutes in the same direction. The third gene that is under control of the Lac operon is the Lac A gene which currently has an unknown function.
Next to the Lac operon are three additional sites: O, P, and I. The O site is for the operator, which is a regulatory site. Proteins bind to this site to halt transcription, which will be discussed shortly. The P site is the promoter region, the site in which RNA polymerase can bind to transcribe the gene. In front of the promoter region there is an I gene that is used to create a suppressor protein that can bind to operator region to block transcription. This repressor is only produced when lactose is not present in the cell. It would be wasteful for the cell to expend resources to transcribe and translate proteins to breakdown lactose if it was not actually present within the cell.
So how is the Lac operon switched on? A protein called CAP is used as an activator to initiate the transcription of these genes. The CAP protein will only work as an activator if cyclic AMP is not present in the cell. Cyclic AMP works as an allosteric effector of the CAP protein, meaning that if CAP will only bind to DNA is cAMP is present. However, two other conditions must be met for the Lac operon to work: Glucose must be absent in the cell and Lactose must be present. The main purpose of the Lac operon is to produce more glucose when glucose levels are low in the cell. Fructose is also produced which can be used in other pathways to provide fuel for the cell. Overall the Lac operon gives insight about how complex biological systems can be and also how they have been formed to function optimally under specific substrate concentrations within the cell.