An operator is a segment of DNA in a prokaryotic operon that interacts with regulatory proteins to control transcription of adjacent structural genes.
Explanation
The operator is a key component of the operon model of gene regulation proposed by Francois Jacob and Jacques Monod. It is typically located downstream of the promoter and upstream of or overlapping the coding sequences of structural genes. Repressor proteins recognize and bind to specific nucleotide sequences within the operator, forming a physical block that prevents RNA polymerase from initiating or progressing through transcription. Binding of a repressor is often controlled allosterically by small metabolites that act as inducers or corepressors. When an inducer binds to a repressor, it causes a conformational change that reduces the repressor’s affinity for the operator and permits gene transcription. Conversely, a corepressor enables a repressor to bind the operator and silence gene expression. Operators thus allow prokaryotic cells to respond rapidly to environmental changes by modulating the expression of metabolic pathways.
Key examples and roles
In the lac operon of Escherichia coli, the LacI repressor binds to the lac operator to prevent transcription of genes required for lactose uptake and metabolism. When lactose is present, it is converted to allolactose, which binds LacI and prevents it from attaching to the operator, allowing transcription. The lac system has three operator sites (O1, O2 and O3) that enhance repression through cooperative binding. In the trp operon, tryptophan acts as a corepressor; when abundant, it binds the TrpR repressor, enabling it to bind the trp operator and shut down synthesis of tryptophan biosynthetic enzymes. Bacteriophage lambda uses operator sites to control the balance between lysogenic and lytic replication, with lambda repressor binding to specific operators to maintain latency. These examples illustrate how operators integrate signals to regulate gene expression.
Operators remain a central concept in bacterial genetics, illustrating how simple DNA elements and proteins coordinate metabolic responses. Understanding operator–repressor interactions has informed molecular biology and biotechnology, including the design of inducible expression systems.
Related Terms: Operon, Promoter, Structural gene, Repressor, Inducer