Abstract
We have engineered a monomeric blue non-fluorescent chromoprotein called Ultramarine (fluorescence quantum yield, 0.001; ε(585 nm), 64,000 M(-1) x cm(-1)) for use as a Förster resonance energy transfer acceptor for a number of different donor fluorescent proteins. We show its use for monitoring activation of caspase 3 in live cells using fluorescence lifetime imaging. Ultramarine has the potential to increase the number of cellular parameters that can be imaged simultaneously.