DOMINANT-NEGATIVE SUPPRESSION OF ARABIDOPSIS PHOTORESPONSES BY MUTANTPHYTOCHROME-A SEQUENCES IDENTIFIES SPATIALLY DISCRETE REGULATORY DOMAINS IN THE PHOTORECEPTOR

We used the exaggerated short hypocotyl phenotype induced by oat phyto chrome A overexpression in transgenic Arabidopsis to monitor the biolo gical activity of mutant phytochrome A derivatives. Three different mu tations, which were generated by removing 52 amino acids from the N te rminus (DELTAN52), the entire C-terminal domain (DELTAC617), or amino acids 617-686 (DELTA617-686) of the oat molecule, each caused striking dominant negative interference with the ability of endogenous Arabido psis phytochrome A to inhibit hypocotyl growth in continuous far-red l ight (''far-red high irradiance response'' conditions). By contrast, i n continuous white or red light, DELTAN52 was as active as the unmutag enized oat phytochrome A protein in suppressing hypocotyl elongation, while DELTAC617 and DELTA617-686 continued to exhibit dominant negativ e behavior under these conditions. These data suggest that at least th ree spatially discrete molecular domains coordinate the photoregulator y activities of phytochrome A in Arabidopsis seedlings. The first is t he chromophore-bearing N-terminal domain between residues 53 and 616 t hat is apparently sufficient for the light-induced initiation but not the completion of productive interactions with transduction chain comp onents. The second is the C-terminal domain between residues 617 and 1 129 that is apparently necessary for completion of productive interact ions under all irradiation conditions. The third is the N-terminal 52 amino acids that are apparently necessary for completion of productive interactions only under far-red high irradiance conditions and are co mpletely dispensable under white and red light regimes.