Although most plant species from algae to flowering plants use blue light for inducing phototropism and chloroplast movement, many ferns, some mosses, and green algae use red as well as blue light for the regulation of these responses, resulting in better sensitivity at low light levels. During their evolution, ferns have created a chimeric photoreceptor (phy3 in Adiantum) between phytochrome (phy) and phototropin (phot) enabling them to use red light effectively. We have identified two genes resembling Adiantum PHY3, NEOCHROME1 and NEOCHROME2 (MsNEO1 and MsNEO2), in the green alga Mougeotia scalaris, a plant famous for its light-regulated chloroplast movement. Like Adiantum PHY3, both MsNEO gene products show phytochrome-typical bilin binding and red͞far-red reversibility, the difference spectra matching the known action spectra of light-induced chloroplast movement in Mougeotia. Furthermore, both genes rescue red-light-induced chloroplast movement in Adiantum phy3 mutants, indicating functional equivalence. However, the fern and algal genes seem to have arisen independently in evolution, thus providing an intriguing example of convergent evolution. convergent evolution ͉ Mougeotia
Light-driven potassium ion uptake in Hul(hzcterium halohiurn is mediated by bacteriorhodopsin. This uptake is charge-balanced by sodium ions and not by proton release. Light-induced shifts in concentrations of divalent cations were found to be negligible. The transient changes in extracellular pH (alkaline overshoot) can be understood by the concomitant processes of ATP synthesis, proton/ sodium exchange and potassium uptake. The driving force of potassium ion uptake is the membrane potential, no ATP-dependent potassium transport process is found.Fluorescence measurements indicate a high permeability of the membrane to potassium ions compared to sodium ions. Therefore the potassium ion diffusion potential contributes to the membrane potential (about 30 mV/decade) and thereby influences the ATP level. Sudden enhancement of the diffusion potential by the potassium ionophore monactin leads to the expected transient increase in cellular ATP level. Due to the large size (up to 100-fold) of the potassium ion gradient and its high capacity (intracellular concentration up to 3 M) the potassium ion gradient can well serve the cell as a long term storage form of energy.The potential difference across biological membranes (A$,) was initially regarded as a diffusion potential ( A $ d ) carried by the ionic gradients of K' and to a lesser extent by other ions like Na' and Cl-. where Zp is the current density I generated by the electrogenic pump and g , is the membrane conductivity. The total membrane potential A$,,, then is the sum of the diffusion potential and the electrogenic potential.(3) As a consequence, bioenergetic processes which can be driven by membrane potential, such as ATP synthesis, will tend to equilibrate with the electric potential difference carried by ionic gradients and electrogenic pumps. In addition ionic gradients will be influenced by the action of electrogenic pumps.Halobacterium halobium provides a good example of these bioenergetic relationships. Conversion of light energy into chemical energy in this organism is based on the action of the electrogenic proton pump bacteriorhodopsin [5 -71. The proton ejection from the cell creates an increase in membrane potential and an increase of pH difference across the cell mem-
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.
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