XVI International Botanical Congess

The Arabidopsis NPH1 gene has been hypothesized to encode the photoreceptor for phototropism. On irradiation with blue light it's protein product, nph1, becomes auto- phosphorylated on irradiation with blue light as an early step in the phototropism signal-transduction pathway. Autophosphorylation of recombinant nph1 protein produced by a Baculovirus/insect cell system is also induced by blue light. The protein itself binds FMN, and its fluorescence excitation spectrum closely resembles the action spectrum for phototropism. As no other plant proteins are present in the system, we conclude that nph1 is itself the photoreceptor for the reaction, and hence the photoreceptor for phototropism. There are two domains in nph1,LOV1 and LOV2, each of which tightly bind FMN. We are investigating which residues are required for flavin binding, and also early photochemical steps that may be involved in signal transduction. On the basis of the above results,we have named the the photoreceptor holoprotein PHOTOTROPIN.