Lac operon- Definition, structure, Inducers, diagram


The lac operon is a set of genes and regulatory elements that control the expression of enzymes involved in lactose metabolism in the bacterium Escherichia coli. The lac operon consists of three structural genes (lacZ, lacY and lacA), an operator site (Olac), a promoter site (Plac) and a regulator gene (lacI). The structural genes encode proteins that are responsible for transporting, hydrolyzing and modifying lactose, while the regulator gene encodes a protein that can bind to the operator site and inhibit the transcription of the structural genes. The operator site is a DNA sequence that lies between the promoter site and the structural genes and acts as a switch for transcription. The promoter site is a DNA sequence that binds RNA polymerase and initiates transcription of the structural genes. The lac operon is an example of an inducible operon, which means that its expression is normally low or absent but can be turned on by the presence of a specific molecule called an inducer. The main inducer of the lac operon is allolactose, which is a derivative of lactose produced by the enzyme β-galactosidase. Allolactose binds to the repressor protein encoded by lacI and prevents it from binding to the operator site, thus allowing transcription of the structural genes. The lac operon is also regulated by another factor called catabolite activator protein (CAP) or cAMP receptor protein (CRP), which enhances transcription in response to low glucose levels. CAP/CRP binds to a site near the promoter site and facilitates the binding of RNA polymerase to Plac. The lac operon is therefore subject to both negative and positive regulation by different molecules depending on the availability of lactose and glucose in the environment.