Long emission wavelength genetic indicator of protease activity Fusion of T-Sapphire to DsRed. The GFPs are linked by a protease cleavage site specific for enterokinase (EK). A shows the excitation and emission spectra of the donor T-Sapphire (green) and DsRed (red) individually. B shows the emission spectrum of the fusion protein before (thick solid line) and at 10 min, 30 min, 1h and 1h 30 min (dotted lines) after digestion with Enterokinase. Digestion by enterokinase completely separates the fluorophores from each other after 1hr 30mins. Excitation of the fusion protein was at 399 nm. Note the complete separation of the donor and acceptor spectra after digestion with enterokinase.