Chen et al. refactored two-component systems (TCSs) to show response regulators (RRs) and histidine kinases (HKs) as concentration-dependent activators and inhibitors. They engineered ultrasensitive TCS sensors, a synergistic sensing system combining the best features of one-component systems and TCSs, and an RR-based low-noise amplifier for upgrading biosensor performance, demonstrating high programmability of TCSs for customizing gene expression and optimizing biosensors.