The Distance Between the Phytochrome Chromophore and the N‐terminal Chain Decreases During Phototransformation. A Novel Fluorescence Energy Transfer Method Using Labeled Antibody Fragments

A novel antibody-fluorescence method has been developed to elucidate the chromophore topography in phytochrome as it undergoes a photochromic transformation. Förster energy transfer from N-terminal bound, fluorescently labeled Oat-25 Fab antibody fragments to the phytochrome chromophore was measured. The results suggest that the chromophore moves relative to the N-terminus upon the Pr-->Pfr phototransformation. This conclusion is consistent with previous models which have proposed a reorientation and an interaction of the Pfr chromophore with the N-terminus. The method described appears to be the first study of a Förster energy transfer measurement using a donor-label attached to a Fab fragment of a photosensor protein.