AT1.03NL is a genetically encoded fluorescent ATP sensor that is optimized for use at relatively low temperatures (20-30[o]C). It consists of an ATP-binding sequence (derived from the F[]F[]-ATP synthase ε subunit encoded by the Bsub\atpI gene) sandwiched between two different fluorophores (the mseCFP cyan fluorescent protein and monomeric Venus carrying the A206K amino acid substitution). In the absence of binding to ATP, low fluorescence resonance energy transfer (FRET) efficiency is seen. Upon binding ATP, a change in conformation of the ATP-binding sequence brings the two fluorophores closer together, increasing FRET efficiency (FBrf0222455). For detailed kinetic and spectral properties, see FBrf0222455.