Abstract: | Abstract— Circular dichroic properties of native, 124 kDa phytochrome from etiolated Avena sativa seedlings have been examined and compared with those of degraded phytochrome (118/114 kDa). The CD spectrum of the Pr form of 124 kDa phytochrome does not differ significantly in the visible region from that of 118/114 kDa Pr. In contrast, the CD spectrum of the Pfr form of 124 kDa phytochrome differs from that of the 118/114 kDa species in the far-red, red and blue regions of the spectrum. This result confirms that the NH2-terminal polypeptide segment has a critical role in chromophore-protein interaction in the Pfr but not in the Pr form. In the UV region, 124 kDa phytochrome exhibits a photoreversible difference between the CD spectra of Pr and Pfr, whereas no such difference is observed for 118/114 kDa preparations. These data suggest a possible photoreversible change in secondary structure of the 124 kDa phytochrome polypeptide that requires the presence of the 6/10 kDa NH2-terminal domain to occur. |