Fig. (3).

Simple biological sensors could be built from protein or RNA components. (a) A two component protein system consisting of a sensor-kinase and a response regulator. In this example, a membrane bound sensor-kinase phosphorylates the response regulator only in the presence of ligand. The phosphorylated response regulator then activates gene expression. (b) An example of one type of riboswitch system that directly controls protein production in response to the presence or absence of ligand. Upon the binding of ligand, the ribosome binding site (RBS) is unmasked, thereby allowing for ribosome binding and thus protein synthesis. Both systems shown here illustrate an "on" switch; however, analogous "off" switches also exist.