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

Abstract

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.

Cryptogam phytochromes

“…The level declined to less than 10% of the initial value after 5 min red-light irradiation, and the red-light effect could be reversed by a subsequent far-red light treatment (Winands & Wagner 1996). These data, together with the presence of a single phytochrome gene in this alga, indicate that phytochrome gene expression is autoregulated (Winands & Wagner 1996). Phytochrome transcript levels in Mesotaenium are also light-regulated, although it is not known which gene is involved, as all the genes are very similar in sequence, and RNA blot analysis cannot distinguish the expression pattern of each gene (Lagarias et al 1995).…”

“…However, no hydrophobie characteristics indicating transmembrane domains were found in the deduced amino acid sequence (Winands & Wagner 1996). Instead, a possible microtubuie-binding domain was found in the C-terminal 16-mer 3-foId repeats in both Mougeotia and Mesotaenium (Winands & Wagner 1996).…”

“…Full sequences of algal phytochromes have been reported from Mesotaenium (Lagarias, Wu & Lagarias 1995) and Mougeotia (Winands & Wagner 1996), both of which showed conventional phytochrome structure. Mesotaenium has at least two very similar phytochrome genes {MESPHYla and MESPHYlb) which may each have two isoforms, all of wbicb have been shown to be expressed (Lagarias et al 1995).…”

“…Mesotaenium has at least two very similar phytochrome genes {MESPHYla and MESPHYlb) which may each have two isoforms, all of wbicb have been shown to be expressed (Lagarias et al 1995). On the other hand, a single phytochrome gene appears to be transcribed in Mougeotia (Winands et al 1992;Winands & Wagner 1996).…”

“…Three introns at the positions known (Hanelt etal. 1992) and MESPHYlb (Lagarias, Wu & Lagarias 1995;Winands & Wagner 1996).…”