Structural studies on the photoreceptor phytochrome: reevaluation of the epitope for monoclonal antibody Z-3B1

Phytochrome of the green alga Mougeotia: cDNA sequence, autoregulation and phylogenetic position

A cDNA clone encoding phytochrome (apoprotein) of the zygnematophycean green alga Mougeotia scalaris has been isolated and sequenced. The clone consisted of 3372 bp, encoded 1124 amino acids, and showed strainspecific nucleotide exchanges for M. scalaris, originating from different habitats. No indication was found of multiple phytochrome genes in Mougeotia. The 5' non-coding region of the Mougeotia PHY cDNA harbours a striking stem-loop structure. Homologies with higher-plant phytochromes were 52-53% for PHYA and 57-59% for PHYB. Highest homology scores were found with lower-plant phytochromes, for example 67% for Selaginella (Lycopodiopsida), 64% for Physcomitrella (Bryopsida) and 73% for Mesotaenium (Zygnematophyceae). In an unrooted phylogenetic tree, the position of Mougeotia PHY appeared most distant to all other known PHYs. The amino acids Gly-Val in the chromophore-binding domain (-Arg-Gly-Val-His-Gly-Cys-) were characteristic of the zygnematophycean PHYs known to date. There was no indication of a transmembrane region in Mougeotia phytochrome in particular, but a carboxyl-terminal 16-mer three-fold repeat in both, Mougeotia and Mesotaenium PHYs may represent a microtubule-binding domain. Unexpected for a non-angiosperm phytochrome, its expression was autoregulated in Mougeotia in a red/far-red reversible manner: under Pr conditions, phytochrome mRNA levels were tenfold higher than under Pfr conditions.

Evidence for the existence of membrane-associated phytochrome in the cell

Comparative fluorescence and photochemical studies of phytochrome in etiolated seedlings of maize and in soluble and membrane-containing fractions isolated from them were carried out. The membrane fractions prepared in the absence of Mg2+ from etiolated coleoptiles contained 13% of total photoreversible phytochrome, which was readily solubilized by mild detergents. Its molecular size was indistinguishable from soluble phytochrome and equal to nondegraded maize phytochrome. Low-temperature fluorescence studies with intact tissue found that the position of the emission maximum at 85 K (lambda max) and the extent of the phototransformation of the red-absorbing form (Pr) into the first stable photoproduct, lumi-R, at 85 K (gamma 1), varied in different parts of etiolated seedlings: lambda max and gamma 1 reached their maximum values in the tips of coleoptiles and roots, 686 nm and 0.30-0.40, whereas the lowest values, 682 nm and ca 0.05, were observed in the root base. These parameters correlated well with those obtained for the pigment in the soluble and membrane-containing fractions: 684 and 680 nm, and 0.33 and 0.06, respectively. The extent of the Pr phototransformation into the far red-absorbing form (Pfr) (gamma 2) did not differ much: values of 0.80-0.85 and 0.70-0.75 correlated with the high and low values of gamma 1. These variations of the parameters were interpreted in agreement with our previous observations in terms of two phytochrome A species whose relative concentrations vary depending on the experimental conditions--the longer wavelength bulk light-labile species with high gamma 1 (Pr'), and the shorter wavelength minor light-stable species with low gamma 1 (Pr").(ABSTRACT TRUNCATED AT 250 WORDS)

EVIDENCE FOR THE EXISTENCE OF MEMBRANE‐ASSOCIATED PHYTOCHROME IN THE CELL

Abstract Comparative fluorescence and photochemical studies of phytochrome in etiolated seedlings of maize and in soluble and membrane‐containing fractions isolated from them were camed out. The membrane fractions prepared in the absence of Mg2+ from etiolated coleoptiles contained 13% of total photoreversible phytochrome, which was readily solubilized by mild detergents. Its molecular size was indistinguishable from soluble phytochrome and equal to nondegraded maize phytochrome. Low‐temperature fluorescence studies with intact tissue found that the position of the emission maximum at 85 K (λmax) and the extent of the phototransformation of the red‐absorbing form (Pr) into the first stable photoproduct, lumi‐R, at 85 K (γ1), varied in different parts of etiolated seedlings: λmax and γ1 reached their maximum values in the tips of coleoptiles and roots, 686 nm and 0.30–0.40, whereas the lowest values, 682 nm and ca 0.05, were observed in the root base. These parameters correlated well with those obtained for the pigment in the soluble and membrane‐containing fractions: 684 and 680 nm, and 0.33 and 0.06, respectively. The extent of the Pr phototransformation into the far red‐absorbing form (Pfr) (γ2) did not differ much: values of 0.80–0.85 and 0.70–0.75 correlated with the high and low values of γ1. These variations of the parameters were interpreted in agreement with our previous observations in terms of two phytochrome A species whose relative concentrations vary depending on the experimental conditions—the longer wavelength bulk light‐labile species with high γ1 (Pr″), and the shorter wavelength minor light‐stable species with low γ1 (Pr″). Close similarity between Pr’and the soluble phytochrome and between Pr″ and the membrane‐bound phytochrome points to the possible origin of the native Pr’and PrPrime; species, thus providing evidence for the existence of membrane‐bound pigment in the cell.

Mosses do express conventional, distantly B-type-related phytochromes. Phytochrome of Physcomitrella patens (Hedw.)

We have screened a cDNA library of the moss Physcomitrella patens (Hedw.) for phytochrome sequences. The isolated sequences turned out to encode a phytochrome dissimilar to the phytochrome type postulated for the moss Ceratodon [(1992) Plant Mol. Biol. 20, 1003-1017] Physcomitrella phytochrome was completely alignable to fern phytochrome (Selaginella) and phytochromes of higher plants. The frequency of clones encoding this phytochrome indicated that a Ceratodon-like type should only be expressed, if at all, with lower frequencies than the sequenced phytochrome cDNA. Sequence differences between lower plant phytochromes are small as compared to phytochrome types of higher plants.