Fig. 1.

Measuring the concentration of a signaling molecule by a biological sensor, which in turn controls downstream events, is a generic task. Here, several examples are depicted schematically for E. coli. Binding of attractant/repellent molecules to a surface receptor complex modulates the rate of autophosphorylation of the associated kinase. This change in kinase activity results in a corresponding concentration change of the internal signaling molecule, CheY∼P, that controls the direction of flagellar motor rotation. Also shown is transcription initiation, where the promoter region can be regarded as a sensor for transcription factors (TF). These proteins, whose concentrations vary depending on the cell cycle and external cues, determine whether or not RNA polymerase (RNAP) turns on a gene.