10.6:
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.
10.6:
Operons
Prokaryotes can control gene expression through operons—DNA sequences consisting of regulatory elements and clustered, functionally related protein-coding genes. Operons use a single promoter sequence to initiate transcription of a gene cluster (i.e., a group of structural genes) into a single mRNA molecule. The terminator sequence ends transcription. An operator sequence, located between the promoter and structural genes, prohibits the operon’s transcriptional activity if bound by a repressor protein. Altogether, the promoter, operator, structural genes, and terminator form the core of an operon.
Operons are usually either inducible or repressible. Inducible operons, such as the bacterial lac operon, are normally “off” but will turn “on” in the presence of a small molecule called an inducer (e.g., allolactose). When glucose is absent, but lactose is present, allolactose binds and inactivates the lac operon repressor—allowing the operon to generate enzymes responsible for lactose metabolism.
Repressible operons, such as the bacterial trp operon, are usually “on” but will turn “off” in the presence of a small molecule called a corepressor (e.g., tryptophan). When tryptophan—an essential amino acid—is abundant, tryptophan binds and activates the trp repressor—preventing the operon from making enzymes required for its synthesis.
Operons may also be constitutively (i.e., continuously) active. For example, bacterial ribosomal RNA (rRNA) operons are always “on” because rRNAs are constantly required for translation.
Other regulatory elements contribute to an operon’s coordinated gene expression as well. Regulatory genes encode transcriptional activator or repressor proteins. The lacI and trpR genes, for example, encode for their respective operon’s repressors. Additional regulatory sequences, such as the lac operon’s catabolite activator protein (CAP) binding site, provide binding sites for other activators or repressors. For instance, when glucose is low, a signaling molecule (i.e., cyclic AMP) activates CAP—permitting it to bind the CAP site, recruit RNA polymerase, and initiate lac operon transcription.
Suggested Reading
Osbourn, Anne E., and Ben Field. "Operons." Cellular and Molecular Life Sciences 66, no. 23 (2009): 3755-3775. [Source]