Schematic of the genetically-encodable BRET molecular tension sensor (BRET-TS). (A) NanoLuc, the energy donor, is excited via addition of furimazine (Fz) in the presence of oxygen. In the absence of force (f), resonance energy transfer occurs to the fluorescent acceptor, mNeonGreen. With applied force, the donor-acceptor pair is separated, reducing resonance energy transfer. Unloaded spectral and resonance energy transfer properties of (B) BRET-TS in comparison to (C) FRET-based TSMod. Spectral components and resulting best-fit additive spectrum depicted. Axes in units of luminescence normalized at 460 nm and relative fluorescence units (RFU).