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Science 299 (5608): 902-906

Copyright © 2003 by the American Association for the Advancement of Science

GUN4, a Regulator of Chlorophyll Synthesis and Intracellular Signaling

Robert M. Larkin,12 Jose M. Alonso,2 Joseph R. Ecker,2 Joanne Chory12*

Nuclear genes control plastid differentiation in response to developmental signals, environmental signals, and retrograde signals from plastids themselves. In return, plastids emit signals that are essential for proper expression of many nuclear photosynthetic genes. Accumulation of magnesium-protoporphyrin IX (Mg-Proto), an intermediate in chlorophyll biosynthesis, is a plastid signal that represses nuclear transcription through a signaling pathway that, in Arabidopsis, requires the GUN4 gene. GUN4 binds the product and substrate of Mg- chelatase, an enzyme that produces Mg-Proto, and activates Mg-chelatase. Thus, GUN4 participates in plastid-to-nucleus signaling by regulating Mg-Proto synthesis or trafficking.

1 Howard Hughes Medical Institute,
2 Plant Biology Laboratory, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA.
*   To whom correspondence should be addressed. E-mail: chory{at}

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