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. Author manuscript; available in PMC: 2018 Aug 8.
Published in final edited form as: Essays Biochem. 2018 Apr 13;62(1):95–107. doi: 10.1042/EBC20170011

Figure 1. GENOMES UNCOUPLED (GUN) chloroplast proteins regulate the expression of nuclear genes.

Figure 1

(A) Tetrapyrroles are synthesized in the plastid from glutamyl-tRNA precursors. At a key branch point, PP IX is converted either into haem by the Fe chelatases or into Mg PP IX by the Mg chelatase complex (composed of ChlH = GUN5, ChlI, ChlD, and GUN4), which is then dedicated to chlorophyll biosynthesis. FC2-synthesized haem remains in the chloroplast, where it associates with pETC proteins. FC1 synthesizes haem for the rest of the cell. Nuclear genes that encode photosynthesis-associated proteins (PhANGs) are positively regulated by FC1-generated haem and/or negatively regulated by Mg PP IX accumulation. (B) GUN1 integrates diverse signals to control nuclear gene expression. Severe disruption of chloroplast function, such as inhibition of plastid translation or oxidative stress caused by inhibition of carotenoid biosynthesis, triggers a retrograde signalling pathway from the chloroplast to the nucleus that requires GUN1. GUN1 promotes the activity of ABI4, a transcription factor that repress PhANG expression, and represses transcription of GLK1/2, transcription factors that promote photomorphogenesis by antagonizing PIF transcription factors.