Two-component sensors

Taken in isolation the simplest bacterial sensing system allows the cell to adapt to an external factor through means of two protein components that are: a sensor kinase and a response regulator. The sensor kinase is buried so which there is an environmental signal binding site outside the cell membrane and an ATP binding cassette on the cytoplasmic side of the cell membrane in Figure. Binding of an external molecule causes conformational change across the whole protein and the protein on the inner membrane is now able to transfer a phosphate group from ATP to a relay protein. In some cases this relay protein is a transcriptional regulator.

 
                               Figure:  A schematic of a two-component regulatory system