PHOTOACTIVATION TURNS GREEN FLUORESCENT PROTEIN RED

In the few years since its gene was first cloned, the Aequorea victori a green fluorescent protein (GFP) has become a powerful tool in cell b iology, functioning as a marker for gene expression, protein localizat ion and protein dynamics in living cells [1-3], GFP variants with impr oved fluorescence intensity and altered spectral characteristics have been identified, but additional GFP variants are still desirable for m ultiple labeling experiments, protein interaction studies and improved visibility in some organisms [4], In particular, long-wavelength (red ) fluorescence has remained elusive, Here we describe a red emitting, green-absorbing fluorescent state of GFP that is generated by photoact ivation with blue light, GFP can be switched to its red-emitting state easily with a laser or fluorescence microscope lamp under conditions of low oxygen concentration, This previously unnoticed ability enables regional, non-invasive marking of proteins in vivo. In particular, we report here the use of GFP photoactivation to make the first direct m easurements of protein diffusion in the cytoplasm of living bacteria. (C) Current Biology Ltd ISSN 0960-9822.