ABSTRACT
In order to achieve input–output robustness, the protein levels must somehow cancel out in the expression for the input–output curve. Remarkably, such robustness occurs in bacterial two-component systems – a class of thousands of systems. A robust input–output curve is crucial in two-component system because the response to osmotic pressure had better be accurate, to avoid the cell bursting or imploding. Robustness in the present mechanism seems to require the combined effects of several biochemical features. The black box also points to system characteristics that rule out robustness. The black box suggests that robustness of the present type cannot generally occur if there is more than one reaction that introduces phosphoryl groups into the system. The mechanism of robustness by bifunctional enzymes that catalyze antagonistic reactions applies to other systems and organisms. In each case, a bifunctional enzyme is at the core of the mechanism, and additional biochemical features combine to allow robustness.
