In positive control, gene expression is enhanced by the presence of a regulatory protein (in contrast to negative control, where its action is reduced). The arabinose operon of E. coli is a classic example. The regulator gene araC produces a repressor (P₁) in the absence of the substrate arabinose. If arabinose is available, P₁ is converted to P₂ (by a conformational change), which is an activator of transcription in the presence of cyclic adenosine monophosphate (cAMP). While the negative control (P₁) is correlated with a low demand for expression, the activator (P₂) appears in response to the demand for high level of expression. In general cases, the addition of an activator protein to the DNA makes possible normal transcription but adding a special ligand to the system removes the activator and the gene is turned off.  arabinose operon,  negative control,  Lac operon,  autoregulation,  catabolite activator protein,  regulation of gene activity