GUN4 is required for posttranslational control of plant tetrapyrrole biosynthesis.
In aerobic photosynthetic organisms, GUN4 binds the chlorophyll intermediates protoporphyrin and Mg protoporphyrin, stimulates Mg chelatase activity, and is implicated in plastidic retrograde signaling. GUN4 expression is most abundant in young and greening tissues and parallels the activity of 5-aminolevulinic acid (ALA) ALA and Mg porphyrin biosynthesis during photoperiodic growth. We explored function and mode of action of GUN4 using GUN4-deficient and overexpressing plants. GUN4 overexpression leads to a general activation of the enzymes of chlorophyll biosynthesis. During photoperiodic growth GUN4 deficiency prevents ALA synthesis and chlorophyll accumulation. All these metabolic changes do not correlate with altered gene expression or changes of protein abundance in tetrapyrrole biosynthesis. While ALA feeding fails to compensate GUN4 deficiency during light-dark growth, this approach results in chlorophyll accumulation under continuous dim light. A new model defines the involvement of GUN4 in posttranslational regulation of ALA and Mg porphyrin synthesis, to sustain chlorophyll synthesis, namely under varying environmental conditions.
Peter
Enrico
E
Institute of Biology/Plant Physiology, Humboldt University, Philippstr. 13, Building 12, D 10115 Berlin, Germany.
Grimm
Bernhard
B
D000622:Aminolevulinic Acid / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D002734:Chlorophyll / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism*; D008029:Lighting; D010788:Photosynthesis; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D010944:Plants / Q000235:genetics / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D036226:Seedlings / Q000254:growth & development / Q000528:radiation effects; D045725:Tetrapyrroles / Q000096:biosynthesis* / Q000378:metabolism; D014158:Transcription, Genetic
D029681:Arabidopsis Proteins; C472017:GUN4 protein, Arabidopsis; D047908:Intracellular Signaling Peptides and Proteins; D010940:Plant Proteins; D045725:Tetrapyrroles; D002734:Chlorophyll; D000622:Aminolevulinic Acid