In Arabidopsis, GUN4 is required for the functioning of the plastid mediated repression of nuclear transcription that is involved in controlling the levels of magnesium- protoporphyrin IX. GUN4 binds the product and substrate of Mg-chelatase, an enzyme that produces Mg-Proto, and activates Mg-chelatase. GUN4 is thought to participates in plastid-to-nucleus signaling by regulating magnesium-protoporphyrin IX synthesis or trafficking.

1: GUN4, a regulator of chlorophyll synthesis and intracellular signaling. Larkin RM, Alonso JM, Ecker JR, Chory J; Science 2003;299:902-906. PMID:12574634