Decreased and increased expression of the subunit CHL I diminishes Mg chelatase activity and reduces chlorophyll synthesis in transgenic tobacco plants.
The chelation of Fe2+ and Mg2+ ions forms protoheme IX and Mg-protoporphyrin IX, respectively, and the latter is an intermediate in chlorophyll synthesis. Active magnesium protoporphyrin IX chelatase (Mg-chelatase) is an enzyme complex consisting of three different subunits. To investigate the function of the CHL I subunit of Mg-chelatase and the effects of modified Mg-chelatase activity on the tetrapyrrole biosynthetic pathway, we characterized N. tabacum transformants carrying gene constructs with the Chl I cDNA sequence in antisense and sense orientation under the control of the CaMV 35S promoter. Both elevated and diminished levels of Chl I mRNA and Chl I protein led to reduced Mg-chelatase activities, reflecting a perturbation of the assembly of the enzyme complex. The transformed plants did not accumulate the substrate of Mg-chelatase, protoporphyrin IX, but the leaves contained less chlorophyll and possessed increased chlorophyll a/b ratios, as well as a deficiency of light-harvesting chlorophyll binding proteins of photosystems I and II. The expression and activity of several tetrapyrrolic enzymes were reduced in parallel to lower the Mg-chelatase activity. Consistent with the lower chlorophyll contents, the rate-limiting synthesis of 5-aminolevulinate was also decreased in the transgenic lines analyzed. The consequence of reduced Mg-chelatase on early and late steps of chlorophyll synthesis, and on the organization of light harvesting complexes is discussed.
Papenbrock
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J
Institut für Pflanzengenetik und Kulturpflanzenforschung, Corrensstrasse 3, 06466 Gatersleben, Germany.
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D000622:Aminolevulinic Acid / Q000378:metabolism; D015152:Blotting, Northern; D015153:Blotting, Western; D020134:Catalytic Domain; D002734:Chlorophyll / Q000096:biosynthesis* / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D008190:Lyases / Q000378:metabolism / Q000502:physiology*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D011524:Protoporphyrins / Q000096:biosynthesis; D011758:Pyrroles / Q000378:metabolism; D045725:Tetrapyrroles; D014026:Nicotiana / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*
D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D011524:Protoporphyrins; D011758:Pyrroles; D045725:Tetrapyrroles; D002734:Chlorophyll; D000622:Aminolevulinic Acid; C028025:protoporphyrin IX; D008190:Lyases; C086788:magnesium chelatase