We can use the naturally occurring mechanisms of controlled mRNA transcription, repressors, cooperative binding, and the degradation of mRNA and proteins as a way to implement a logical inverter.
The ``signals'' in our logic system consist of concentrations of specific DNA binding proteins, which act as repressors. These concentrations can be thought of as a simple integer count of how many protein molecules of a particular type exist within a single cell.
The ``inverters'' in our logic system consist of genes--specific DNA coding regions, along with their control sequences--which code for the production of specific proteins. Normally, the coded proteins are themselves DNA binding proteins, which are used as inputs to other such inverters. They could, of course, code for enzymes which effect some other action within the cell, such as motion, illumination, or chemical reactions. Similarly, the input of our gates could consist, not of the output of another logic gate, but of a sensor which creates a DNA binding protein in response to illumination, a chemical in the environment, or the concentration of specific intracellular chemicals.
One additional feature, also present in naturally occurring transcription control mechanisms, is used in our gate to control the output level of the DNA binding protein product. Specifically, the presence of large concentrations of DNA binding protein produced by a particular gene is used to inhibit the transcription of more copies of its mRNA. This results in a predictable amount of the gene product when the gene is turned on.
Logic functions are performed in this model by constructing multiple inverters (genes), each having the same binding protein as a product, but with different control inputs. In many respects, this is similar to the I2L integrated circuit logic family.