Photoreversible absorption change and domain structure of phytochrome

A phytochrome from the fern Adiantum with features of the putative photoreceptor NPH1

In plant photomorphogenesis, it is well accepted that the perception of red/far-red and blue light is mediated by distinct photoreceptor families, i.e., the phytochromes and blue-light photoreceptors, respectively. Here we describe the discovery of a photoreceptor gene from the fern Adiantum that encodes a protein with features of both phytochrome and NPH1, the putative blue-light receptor for second-positive phototropism in seed plants. The fusion of a functional photosensory domain of phytochrome with a nearly full-length NPH1 homolog suggests that this polypeptide could mediate both red/far-red and blue-light responses in Adiantum normally ascribed to distinct photoreceptors.

A monoclonal antibody affects chromophore apoprotein interactions of phytochrome A

A monoclonal antibody designated Mep-1 was raised against phytochrome A from pea (Pisum sativum L.). The binding of this antibody (class IgG1) to partially degraded phytochrome (114 kDa) caused a considerable increase in the far-red peak at the red-light-induced stationary state. The effect reached a plateau value when the antibody and phytochrome were present in approximately equimolar amounts. The dark transformation of the far-red-light-absorbing form to the red-light-absorbing form of the 114 kDa phytochrome was inhibited by the addition of the antibody. However, binding of the antibody to the undegraded 121 kDa phytochrome had no effects on the spectrum of the red-light-induced steady state. The site at which the antibody bound to phytochrome was determined to be between amino acid residues 256 and 383 of pea phytochrome A. This is the first report of a monoclonal antibody that enhances the far-red absorption of phytochrome in the red-light-induced photostationary state.

The structure and function of phytochrome A: the roles of the entire molecule and of its various parts

Phytochrome A is readily cleavable by proteolytic agents to yield an amino-terminal fragment of 66 kilodalton (kDa), which consists of residues 1 to approximately 600, and a dimer of the carboxy-terminal 55-kDa fragment, from residue 600 or so to the carboxyl terminus. The former domain, carrying the tetrapyrrole chromophore, has been studied extensively because of its photoactivity, while less attention has been paid to the non-chromophoric portion until quite recently. However, the evidence gathered to date suggests that this domain is also of great improtance. We present here a review of the structure and the biochemical and physiological functions of the two domains, of parts of these domains, and of the cooperation between them.

Photoreversible change in the conformation of phytochrome as probed with a covalently bound fluorescent sulfhydryl reagent, N-(9-acridinyl)maleimide

A fluorescent SH reagent, N-(9-acridinyl)maleimide (NAM), was used as a fluorescent probe to detect changes in the conformation of phytochrome from pea (Pisum sativum) seedlings during photoconversion between the red-light absorbing form (Pr) and the far-red-light absorbing form (Pfr). NAM-Cys conjugates emitted weaker fluorescence in non-polar solvent than in polar solvent. The fluorescence intensity (FI) of NAM-phytochrome conjugates depended on the absorbing form of phytochrome: the FI of NAM-Pr was greater than that of NAM-Pfr, indicating that the Cys residues modified by NAM were in more hydrophobic environment in Pfr than in Pr. The FI of the conjugate prepared from a red-light-irradiated sample of phytochrome was greater than that for the conjugate prepared from Pr, indicating that more Cys residues were modifiable in Pfr than Pr. The fluorescence polarization of the conjugate (0.0251) indicates that the modified Cys residues may be located at the surface of the phytochrome molecule. The FI of phytochrome conjugates with 8-anilinonaphthalene-1-sulfonate (ANS) did not change with the photoconversion of phytochrome. The FI of a mixture of ANS and phytochrome increased upon the first photoconversion of Pr to Pfr. However, it did not change upon subsequent photoconversion between Pr and Pfr. These results suggest that the initial increase in FI may have resulted from the binding of additional ANS to Pfr and that the microenvironment of bound ANS may not be influenced by the photoconversion of phytochrome.