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Molecular Plant Advance Access originally published online on September 2, 2009
Molecular Plant 2009 2(6):1198-1210; doi:10.1093/mp/ssp072
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© The Author 2009. Published by the Molecular Plant Shanghai Editorial Office in association with Oxford University Press on behalf of CSPP and IPPE, SIBS, CAS.

GUN4 Is Required for Posttranslational Control of Plant Tetrapyrrole Biosynthesis

Enrico Peter and Bernhard Grimm1

Institute of Biology/Plant Physiology, Humboldt University, Philippstr. 13, Building 12, D 10115 Berlin, Germany

1 To whom correspondence should be addressed. E-mail Bernhard.grimm{at}rz.hu-berlin.de.

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.

Key Words: Tetrapyrrole biosynthesis • chlorophyll • Mg porphyrin branch • photo-oxidation • chloroplasts


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