GUN4-Porphyrin Complexes Bind the ChlH/GUN5 Subunit of Mg-Chelatase and Promote Chlorophyll Biosynthesis inArabidopsis 

Abstract

The GENOMES UNCOUPLED4 (GUN4) protein stimulates chlorophyll biosynthesis by activating Mg-chelatase, the enzyme that commits protoporphyrin IX to chlorophyll biosynthesis. This stimulation depends on GUN4 binding the ChlH subunit of Mg-chelatase and the porphyrin substrate and product of Mg-chelatase. After binding porphyrins, GUN4 associates more stably with chloroplast membranes and was proposed to promote interactions between ChlH and chloroplast membranesthe site of Mg-chelatase activity. GUN4 was also proposed to attenuate the production of reactive oxygen species (ROS) by binding and shielding light-exposed porphyrins from collisions with O 2 . To test these proposals, we first engineered Arabidopsis thaliana plants that express only porphyrin binding-deficient forms of GUN4. Using these transgenic plants and particular mutants, we found that the porphyrin binding activity of GUN4 and Mg-chelatase contribute to the accumulation of chlorophyll, GUN4, and Mg-chelatase subunits. Also, we found that the porphyrin binding activity of GUN4 and Mgchelatase affect the associations of GUN4 and ChlH with chloroplast membranes and have various effects on the expression of ROS-inducible genes. Based on our findings, we conclude that ChlH and GUN4 use distinct mechanisms to associate with chloroplast membranes and that mutant alleles of GUN4 and Mg-chelatase genes cause sensitivity to intense light by a mechanism that is potentially complex.

Arabidopsis Defense against Botrytis cinerea: Chronology and Regulation Deciphered by High-Resolution Temporal Transcriptomic Analysis

“…Downregulation of chlorophyll biosynthesis in comparison to the uninfected controls appears to start around 14 HAI but "chlorophyll biosynthesis" is also an overrepresented GO term in clusters whose expression begins to drop at 18 and 20 HAI (clusters 1, 2, and 8). The genes decreasing in expression encode many enzymes required for chlorophyll biosynthesis (such as HEME1, HEME2, and a subunit of Mg-chelatase) as well as GUN4, a regulatory protein that promotes chlorophyll biosynthesis by binding to Mg-chelatase (Adhikari et al, 2011). Chloroplast organization and biogenesis is also overrepresented in cluster 9, containing genes decreasing in expression at 18 HAI.…”

Ethylene-orchestrated circuitry coordinates a seedling’s response to soil cover and etiolated growth

“…HEMA1 is known to encode the first committed enzyme of the chlorophyll biosynthesis pathway of higher land plants (Fig. 6A) (51)(52)(53), whereas genomes uncoupled 4/5 (GUN4/GUN5) are known to determine the branch pathways leading to Pchlide/chlorophyll synthesis (54)(55)(56). As a result of chlorophyll synthesis occurring in the dark, a pool of Pchlide accumulates in the etioplasts.…”

Influence of plastids on light signalling and development

“…Norflurazon specifically inhibits phytoene desaturase, which is required for carotenoid biosynthesis and chloroplast biogenesis [17]. gun alleles either attenuate negative regulators or promote positive regulators of PhANG expression [5,18,[20][21][22][23][24][25].…”