Abstract
In higher plants, phytochromes have a chromophore, phytochromobilin (PΦB) which can change conformation reversibly on absorbance of red- and far red-light. We have identified the Arabidopsis HY2 gene encoding phytochromobilin PΦB synthetase. In addition, we isolated the homolog pcyA gene from Synechocystis encoding phycocyanobilin synthetase. To study the light response of phycocyanobilin (PCB)-phytochrome in vivo, we analyzed transgenic Arabidopsis hy2 mutants overexpressing the Synechocystis pcyA gene with a chloroplast transit peptide. PCB-holophytochrome proteins from transgenic plants showed the characteristic absorption spectra shifted toward blue wavelengths compared with the absorption spectra of PΦB-holophytochrome. The transgenic plants complemented the hy2 phyenotype for flowering time and hypocotyl elongation in white light, but transgenic seedlings showed interference of PhyA-mediated inhibition of hypocotyl elongation in continuous far-red light. These results suggest that PCB-phytochrome is partally functional as an alternative PΦB in plants.
