Fig. 1.
Transcription regulation mechanisms of a gene Z. (a) Positive control (activator). In the absence of an inducing signal (input state X = 0), the binding site OA of activator A is free. This causes the gene to be expressed at a low level Z0. When the signal is present (X = 1), OA is bound by the activator A, which causes the gene to be fully expressed (Z = Z1). (b) Negative control (repressor). When the inducer signal is absent (X = 0), repressor binding site OR is bound by repressor R. This causes the gene to be expressed at a low level Z0. When the inducer is present (X = 1), OR is free, which causes the gene to be fully expressed (Z = Z1). (c) Mapping between input states X, binding states Y, and outputs states Z in the case of positive control. Y = 0/1 denotes whether the binding site is free/bound, respectively. The arrow with asymmetric head denotes errors, which affect the free (Y = 0) binding state. (d) Mapping between input states, binding states, and outputs in the case of negative control.