Visualization of NO3⁻/NO2⁻ Dynamics in Living Cells by Fluorescence Resonance Energy Transfer (FRET) Imaging Employing a Rhizobial Two-component Regulatory System.

Nitrate (NO3(-)) and nitrite (NO2(-)) are the physiological sources of nitric oxide (NO), a key biological messenger molecule. NO3(-)/NO2(-) exerts a beneficial impact on NO homeostasis and its related cardiovascular functions. To visualize the physiological dynamics of NO3(-)/NO2(-) for assessing the precise roles of these anions, we developed a genetically encoded intermolecular fluorescence resonance energy transfer (FRET)-based indicator, named sNOOOpy (sensor for NO3(-)/NO2(-) in physiology), by employing NO3(-)/NO2(-)-induced dissociation of NasST involved in the denitrification system of rhizobia. The in vitro use of sNOOOpy shows high specificity for NO3(-) and NO2(-), and its FRET signal is changed in response to NO3(-)/NO2(-) in the micromolar range. Furthermore, both an increase and decrease in cellular NO3(-) concentration can be detected. sNOOOpy is very simple and potentially applicable to a wide variety of living cells and is expected to provide insights into NO3(-)/NO2(-) dynamics in various organisms, including plants and animals.