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Molecular Signal Processing
Bioorganic Chemistry
Biochemistry of Plant Interactions
Cell and Metabolic Biology
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Publications
Land plants evolved from charophytic algae, among which Charophyceae possess the most complex body plans. We present the genome of Chara braunii; comparison of the genome to those of land plants identified evolutionary novelties for plant terrestrialization and land plant heritage genes. C. braunii employs unique xylan synthases for cell wall biosynthesis, a phragmoplast (cell separation) mechanism similar to that of land plants, and many phytohormones. C. braunii plastids are controlled via land-plant-like retrograde signaling, and transcriptional regulation is more elaborate than in other algae. The morphological complexity of this organism may result from expanded gene families, with three cases of particular note: genes effecting tolerance to reactive oxygen species (ROS), LysM receptor-like kinases, and transcription factors (TFs). Transcriptomic analysis of sexual reproductive structures reveals intricate control by TFs, activity of the ROS gene network, and the ancestral use of plant-like storage and stress protection proteins in the zygote.
Publications
The coding sequence of the cyanobacterium Synechocystis sp. strain PCC 6803 slr0095 gene was cloned and functionally expressed in Escherichia coli. The corresponding enzyme was classified as a cation- and S-adenosyl-l-methionine-dependent O-methyltransferase (SynOMT), consistent with considerable amino acid sequence identities to eukaryotic O-methyltransferases (OMTs). The substrate specificity of SynOMT was similar with those of plant and mammalian CCoAOMT-like proteins accepting a variety of hydroxycinnamic acids and flavonoids as substrates. In contrast to the known mammalian and plant enzymes, which exclusively methylate the meta-hydroxyl position of aromatic di- and trihydroxy systems, Syn-OMT also methylates the para-position of hydroxycinnamic acids like 5-hydroxyferulic and 3,4,5-trihydroxycinnamic acid, resulting in the formation of novel compounds. The x-ray structure of SynOMT indicates that the active site allows for two alternative orientations of the hydroxylated substrates in comparison to the active sites of animal and plant enzymes, consistent with the observed preferred para-methylation and position promiscuity. Lys3 close to the N terminus of the recombinant protein appears to play a key role in the activity of the enzyme. The possible implications of these results with respect to modifications of precursors of polymers like lignin are discussed.
Publications
A mutant of Synechocystis sp. strain PCC6803 was obtained by random cartridge mutagenesis, which could not grow at low sodium concentrations. Genetic analyses revealed that partial deletion of the sll 0273 gene, encoding a putative Na+ /H+ exchanger, was responsible for this defect. Physiological characterization indicated that the sll 0273 mutant exhibited an increased sensitivity towards K+ , even at low concentrations, which was compensated for by enhanced concentrations of Na+ . This enhanced Na+ demand could also be met by Li+ . Furthermore, addition of monensin, an ionophore mediating electroneutral Na+ /H+ exchange, supported growth of the mutant at unfavourable Na+ /K+ ratios. Measurement of internal Na+ and K+ contents of wild‐type and mutant cells revealed a decreased Na+ /K+ ratio in mutant cells pre‐incubated at a low external Na+ /K+ ratio, while it remained at the level of the wild type after pre‐incubation at a high external Na+ /K+ ratio. We conclude that the Sll0273 protein is required for Na+ influx, especially at low external Na+ concentrations or low Na+ /K+ ratios. This system may be part of a sodium cycle and may permit re‐entry of Na+ into the cells, if nutrient/Na+ symporters are not functional or operating.
