Os03g0811100

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Photosynthetic organisms exhibit a green color due to the accumulation of chlorophyll pigments in chloroplasts. Mg-protoporphyrin IX chelatase (Mgchelatase) comprises three subunits (ChlH, ChlD and ChlI) and catalyzes the insertion of Mg2+ into protoporphyrin IX, the last common intermediate precursor in both chlorophyll and heme biosyntheses, to produce Mg-protoporphyrin IX (MgProto). Chlorophyll deficiency in higher plants results in chlorina (yellowishgreen) phenotype. The grana are poorly stacked, resulting in the underdevelopment of chloroplasts. In the seedlings fed with aminolevulinate-dipyridyl in darkness, MgProto levels in the chl1 and chl9 mutants decreased up to 25% and 31% of that in wildtype, respectively, indicating that the Mg-chelatase activity is significantly reduced, causing the eventual decrease in chlorophyll synthesis. Furthermore, Northern blot analysis indicated that the nuclear genes encoding the three subunits of Mg-chelatase and LhcpII in chl1 mutant are expressed about 2-fold higher than those inWT, but are not altered in the chl9 mutant. This result indicates that the ChlD subunit participates in negative feedback regulation of plastidto- nucleus in the expression of nuclear genes encoding chloroplast proteins, but not the ChlI subunit.

Function

Rice Chlorina-1 and Chlorina-9 encode ChlD and ChlI subunits of Mg-chelatase, a key enzyme for chlorophyll synthesis and chloroplast development.MgProto accumulated at a lowerlevel in both chl1 and chl9 mutants when the seedlings were fed by ALA-DP in complete darkness, indicating that Mg-chelatase activity is severely reduced, resulting in decrease in chlorophyll synthesis and underdevelopment of chloroplats. In addition, the ChlD subunit of Mg-chelatase also negatively regulates the expression of nuclear genes encoding photosynthetic proteins in chloroplasts. Chl1 and Chl9 genes encode the OsChlD and OsChlI subunits of Mg-chelatase, respectively, that play an important role in chloroplast development as a modulator of MgProto. Northern blots analysis revealed that ChlD subunit participates in the negative feedback regulation for the expression of nuclear genes encoding chloroplast proteins.

Expression

Rice Chl1 and Chl9 genes on chromosome 3 encode ChlD and ChlI,in chl1 mutants, the levels of OsChlH and OsChlI transcripts increased about 2.5-fold and OsChlD transcript levels were about 1.7-fold higher than the wild-type. In the chl9 mutant, however, OsChlH and OsChlD transcript levels were not altered and the OsChlI transcript was also present in the mutant.expression levels of photosynthetic genes such as the nuclear LhcpII genes encoding a Chl-binding thylakoid protein and the chloroplast RbcL gene encoding a stroma protein to determine if the expressions of nuclear and chloroplast genes were affected in the chl1 and chl9 mutants. Functional OsChlD subunit encoded by the Chl1 gene participates in the negative feedback regulation of plastid-to-nucleus signaling mechanisms that control the expression levels of nuclear genes encoding chloroplast proteins. The expression of chloroplast genes like RbcL is not affected by the activity of Mg-chelatase.


Evolution

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Labs working on this gene

Department of Plant Science, Seoul National University

References

1.Haitao Zhang;Jinjie Li;Jeong-Hoon Yoo;Soo-Cheul Yoo;Sung-Hwan Cho;Hee-Jong Koh;Hak Soo Seo;Nam-Chon Paek, Rice Chlorina-1 and Chlorina-9 encode ChlD and ChlI subunits of Mg-chelatase, a key enzyme for chlorophyll synthesis and chloroplast development Plant Molecular Biology, 2006, 62(3): 325-337

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