Operons

In prokaryotic cells, an operon is a group of genetic sequences that contain regulatory elements and several protein-coding genes, the structural genes, that are transcribed together. One of the best studied cases is the lac operon in bacteria that contains three genes, lacZ, lacY, and lacA, that encode the enzymes necessary for the metabolism of lactose, along with the promoter, operator, and terminator sequences that regulate the expression of the lac genes.

Normally, when glucose concentration is abundant, the lac repressor protein binds tightly to the operator and prevents transcription of the lac genes by blocking RNA polymerase from binding to the promoter. The lac repressor is constitutively expressed, meaning that the gene encoding it is turned on by default. When glucose concentration is very low, the cell will use lactose as an energy source. Once present inside the cell, some of the lactose is converted to a modified version called allolactose, which is known as the inducer of the lac operon because it binds to and inhibits to the repressor, triggering the expression of the lac genes.

Additionally, with low glucose levels, the amount of the signaling molecule cyclic AMP increases, and binds to the catabolite activator protein, or CAP. Together, they bind to a regulatory sequence, proximally upstream of the promoter and help recruit RNA polymerase to significantly increase transcription. During transcription, a single mRNA strand is produced and released when the polymerase reaches the terminator sequence. From this mRNA, the three proteins that are necessary to process lactose are translated.