ARABIDOPSIS NPH1 - A FLAVOPROTEIN WITH THE PROPERTIES OF A PHOTORECEPTOR FOR PHOTOTROPISM

The NPH1 gene of Arabidopsis thaliana encodes a 120-kilodalton serine- threonine protein kinase hypothesized ta function as a photoreceptor f or phototropism. When expressed in insect cells, the NPH1 protein is p hosphorylated in response to blue Light irradiation. The biochemical a nd photochemical properties of the photosensitive protein reflect thos e of the native protein in microsomal membranes. Recombinant NPH1 nonc ovalently binds flavin mononucleotide, a likely chromophore for Light- dependent autophosphorylation. The fluorescence excitation spectrum of the recombinant protein is similar to the action spectrum for phototr opism, consistent with the conclusion that NPH1 is an autophosphorylat ing flavoprotein photoreceptor mediating phototropic responses in high er plants.