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The lactose operon (lac operon) is an operon required for the transport and metabolism of lactose in E. coli and many other enteric bacteria.Although glucose is the preferred carbon source for most enteric bacteria, the lac operon allows for the effective digestion of lactose when glucose is not available through the activity of β-galactosidase. [1]
The lac operon of the model bacterium Escherichia coli was the first operon to be discovered and provides a typical example of operon function. It consists of three adjacent structural genes, a promoter, a terminator, and an operator. The lac operon is regulated by several factors including the availability of glucose and lactose.
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Absence of glucose will "turn off" catabolite repression. When glucose levels are low, the phosphorylated form of EIIA accumulates and consequently activates the enzyme adenylyl cyclase, which will produce high levels of cAMP. cAMP binds to catabolite activator protein (CAP) and together they will bind to a promoter sequence on the lac operon ...
The lac operon in the prokaryote E. coli consists of genes that produce enzymes to break down lactose. Its operon is an example of a prokaryotic silencer. The three functional genes in this operon are lacZ, lacY, and lacA. [6] The repressor gene, lacI, will produce the repressor protein LacI which is under allosteric regulation.
One known example of these transport proteins is the melibiose, or melB, carrier protein derived from the melB gene of E. coli. [3] This strain of beta-galactoside permease is known it transport melibiose and other galactosides across the cell membrane using hydrogen , sodium , or lithium ions in cotransport .
The enzyme's role in the classical E.coli lac operon remains unclear. [ 1 ] [ 3 ] However, the enzyme's cellular role may be to detoxify non-metabolizable pyranosides by acetylating them and preventing their reentry into the cell.
The lacZYA operon houses genes encoding proteins needed for lactose breakdown. [2] The lacI gene codes for a protein called "the repressor" or "the lac repressor", which functions to repressor of the lac operon. [2] The gene lacI is situated immediately upstream of lacZYA but is transcribed from a lacI promoter. [2]