Prof. Rüdiger HellMolecular Biology of Plants
The Department of Molecular Biology of Plants investigates the primary metabolism of sulfur and the the role of amino-terminal protein acetylation in plants. Its teaching activities include molecular, genetics, biochemistry, physiology and biotechnology of model and crop plants. Research and teaching activities are based on state-of-the art facilities and equipment, including the Metabolomics Core Technology Platform.
The regulation of assimilatory sulfate reduction and biosynthesis of cysteine is investigated using the favorite plant of molecular biologists, Arabidopsis thaliana, a member of the Brassicaceae family. Cysteine is of great importance for human and animal nutrition. It is the precursor of donor for all compounds containing reduced sulfur in the cell. Sulfur is indispensable for proteins (methionine, disulfide bridges), electron transport (Fe/S clusters), redox control (glutathione) and cofactors (acetyl coenzyme A, biotin, thiamine, lipoic acid). In plants it additionally contributes to plant defense and secondary metabolism. Reduced sulfur is therefore an indispensable factor for life. Plants as photoautotrophic organisms are, directly or indirectly, the exclusive source of cysteine for animal life.
Investigation of cysteine synthesis is of fundamental interest because it constitutes a central regulator for the entire assimilation pathway of inorganic sulfate and is based on unique biochemical and cell biological properties. Cysteine synthesis is carried out by a bifunctional hetero-decameric protein complex. The cysteine synthase complex and its subunits catalyse the formation of O-acetylserine, the precursor of cysteine, and cysteine itself, and additionally acts as a sensor for sulfide in the cell. Signals via the complex regulate the expression of genes encoding steps upstream in the pathway, notably of sulfate transport at the plasmalemma and reduction of activated sulfate (adenosine phosphosulfate). Cysteine synthase complexes are present in plastids, the cytosol and mitochondria. Our findings using mutants of Arabidopsis indicate specific functions of each of these sites in the synthesis of sulfide, O-acetylserine and cysteine together with regulatory and stress response functions. Further projects address the role of cysteine synthesis for glutathione-mediated redox processes, the link between sulfate reduction with seed storage protein formation and the activation of sulfur metabolism during drought stress.
The N-terminal acetylation of proteins by N-acetyltransferases (NATs) is the other major topic of the department. N-terminal acetylation is probably the most abundant co- or post-translational modification of eukaryotic proteins with more than 80% of proteins being fully or partially acetylated in mammals. Recent findings show a broad spectrum of acetylation including metabolic proteins and the presence of a family of similar NAT complexes in Arabidopsis as compared to yeast and mammals. The remarkable evolutionary conservation of these proteins and their mechanism of action can be well addressed using Arabidopsis as experimental system. The plant approach exceeds yeas and animal systems by its suitability to study cellular and developmental effects of experimentally altered acetylation patterns. Using T-DNA insertion mutants and complementation of Arabidopsis NATs are functionally characterized with respect to target phenotypes.
Regulation of the assimilatory sulfate reduction pathway by internal signals
Besides nitrogen (N) and phosphor, sulfur (S) is the most important macronutrient, which is taken up from the soil. The uptake of S as sulfate is mediated via specific transporters and strictly regulated in higher plants to ensure optimal growth. After the uptake of sulfate in the root, sulfate is transported to the shoot, reduced to sulfide and fixed in the proteinogenic amino acid cysteine. The entire reaction cascade including the uptake, activation, reduction and fixation of S is called assimilatory sulfate reduction pathway (ASRP, Fig 1).
The strict regulation and coordination of ASRP with diverse metabolic pathways, like carbon (C) and N- fixation, implies the presence of internal signals. The C and N containing precursor of cysteine, O-acetylserine (OAS), is supposed to act as such a signal. The sulfate transporters at the plasmalemma of root cells are known to be induced by high intracellular concentration of OAS, which is dependant of the actual S, C and N supply of the plant (Fig 2).
The signal perception and transduction cascade that results in the transcriptional activation of sulfate transporters and key regulators of sulfate reduction are not known at present. A custom made gene array, which covers the complete transcriptome of the ASRP, will be used in combination with N and S-deprivation experiments and knock out mutant of key steps in the ASRP to reveal new targets of regulation by OAS (Fig 3). In addition transgenic plants, which express proteins encoding for key steps in ASRP, will be produced to identify regulatory steps in OAS production and homeostasis.