Escherichia coli adenylate cyclase as a sensor of sugar transport function.

Adenylate cyclase of E. coli is a membrane-bound enzyme the function of which is to synthesize a cofactor for processes that are important in metabolic transitions. The depletion from the environment of a supply of a preferred carbon source dictates the requirement for initiating the synthesis of a new metabolic system; this synthesis will require cAMP. After the adaptation period, the requirement for a high level of synthesis diminishes, resulting in a diminished requirement for cAMP. A mechanism for regulating the activity of adenylate cyclase accomplishes the variation in the required cellular cAMP concentrations. In the absence of a transportable carbon source, adenylate cyclase activity is activated by cellular regulators; when carbon sources are transported, the cellular activators are dissipated, resulting in inhibition of adenylate cyclase activity. This scheme is summarized in Fig. 6. Sugar transport systems fall into two categories: one in which the energy for the process comes from PEP (the PTS) and one in which the energy comes from the proton electrochemical gradient. Adenylate cyclase communicates with both of these systems by interacting with intermediates on the pathway to energy generation for driving these two transport processes. Adenylate cyclase couples indirectly to a large array of sugar-specific transport systems by interacting with intermediates common to all the processes. The net result of this regulatory mechanism is that, without physically communicating with the extracellular environment by spanning the membrane, adenylate cyclase effectively senses the presence of external sugars that interact with cells that have become competent to transport them.