The lac Operon

The lac operon regulatory mechanism allows bacteria to sense external nutrient conditions and, accordingly, regulate gene transcription that assists in the digestion of available nutrients. The lac system produces a switch-like dynamic behavior, where the switching between ‘on’ and ‘off’ states of lac gene transcription depends on the specific thresholds of nutrient concentrations. As lac operon regulation has been extensively studied and is one of the most well-understood gene regulatory systems, it is often used as an example to demonstrate the functional principles of genetic switches. By studying this case, we learn the role of nonlinearity and positive feedback loops in gene regulatory networks, and how gene circuits can generate an all-or-nothing response.