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Diverse microorganisms occupy habitats that are subject to frequent and strong fluctuations. To ensure survival and eventual procreation, microorganisms must hence constantly read out their environment and precisely adjust their cellular physiology in response. In many cases, these processes are mediated by two-component systems (TCS). In their canonical form, TCS comprise a modular sensor histidine kinase (SHK) that senses a chemical stimulus, e.g., the concentration of a metabolite, or a physical stimulus, e.g., light or temperature changes, and phosphorylates a cognate response regulator (RR). In turn, the phospho-RR elicits cellular responses, most often leading to altered gene expression. Despite wide distribution and eminent roles, the signaling mechanism of TCSs is still under debate. Against this backdrop, photoreceptor histidine kinases which sense light in the visible range of the electromagnetic spectrum serve as experimentally tractable paradigms, chiefly because they are soluble proteins and can be triggered by light with ease and supreme precision in time and space. The synthesis of recent data on light-responsive SHKs lets emerge a unifying view of signal transduction. Light-induced rearrangements of a sensor module propagate as torque through coiled-coil linkers to the effector where they induce rotary movements that prompt altered enzymatic output. The mechanistic principles evidenced in light-responsive receptors could widely apply to TCS signaling in general. In addition, light-responsive SHKs and signal receptors serve as genetically encodable tools for the control by light of cellular physiology and behavior in heterologous organisms. This approach, dubbed optogenetics, greatly benefits from the rational engineering of novel light-gated receptors and the discovery of natural photoreceptors with hitherto unknown protein architecture.

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