Abstract
PHY3 is composed of a phytochrome chromophore-binding domain in its N-terminal portion and an almost full-length phototropin in its C-terminal half. Because of its structural features, PHY3 must be able to perceive both red- and blue-light signals simultaneously. When we simultaneously irradiated transgenic Arabidopsis expressing PHY3 with both red and blue light, simultaneous irradiation causes greater phototropic curvature than with red or blue light alone. To address the relationship of this synergistic effect with respect to the PHY3 kinase activity, we examined in vitro phosphorylation analysis of microsomal membranes from a PHY3 expressing line. When we applied red and blue light simultaneously, an obvious increase in the phosphorylation level of the 160-kD protein was detected compared with that of single light irradiation. These results clearly demonstrate that two independent light-sensing systems cooperate within PHY3 and that red light and blue light signals are processed synergistically by this single chromoprotein.
