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.