Abstract
Bacteria need to constantly read out their environment for the rapid adaptation to variable conditions. This crucial need is most frequently served by two-component systems (TCS) which decode environmental stimuli into intracellular responses. As one component, sensor histidine kinases (SHK) control the phosphorylation status of the second component, i.e., the response regulator (RR), which in turn determines the downstream responses. These responses can be highly stringent, acute, and sensitive as SHKs commonly exert both kinase and phosphatase activity towards their RRs. With a bacteriophytochrome TCS as a paradigm, we here interrogate how this catalytic duality underlies the signal responses and their reprogramming. Derivative systems exhibit tenfold enhanced sensitivity to red light, owing to an altered balance between the elementary kinase and phosphatase activities. Modifications of the linker intervening the SHK sensor and catalytic entities likewise tilt this balance and provide novel TCSs with hitherto unavailable, inverted output that increases under red light. Not only do the derivative TCSs support novel applications in synthetic biology and optogenetics, but also, they showcase how deliberate perturbations of the kinase-phosphatase duality underpin the rapid exploration of novel signal-response regimes. Arguably, these aspects equally pertain to the engineering and the natural evolution of TCSs.