title: Mg chelatase in chlorophyll synthesis and retrograde signaling in Chlamydomonas reinhardtii: CHLI2 cannot substitute for CHLI1.
abstract: The oligomeric Mg chelatase (MgCh), consisting of the subunits CHLH, CHLI, and CHLD, is located at the central site of chlorophyll synthesis, but is also thought to have an additional function in regulatory feedback control of the tetrapyrrole biosynthesis pathway and in chloroplast retrograde signaling. In Arabidopsis thaliana and Chlamydomonas reinhardtii, two genes have been proposed to encode the CHLI subunit of MgCh. While the role of CHLI1 in A. thaliana MgCh has been substantially elucidated, different reports provide inconsistent results with regard to the function of CHLI2 in Mg chelation and retrograde signaling. In the present report, the possible functions of both isoforms were analyzed in C. reinhardtii Knockout of the CHLI1 gene resulted in complete loss of MgCh activity, absence of chlorophyll, acute light sensitivity, and, as a consequence, down-regulation of tetrapyrrole biosynthesis and photosynthesis-associated nuclear genes. These observations indicate a phenotypical resemblance of chli1 to the chlh and chld C. reinhardtii mutants previously reported. The key role of CHLI1 for MgCh reaction in comparison with the second isoform was confirmed by the rescue of chli1 with genomic CHLI1 Because CHLI2 in C. reinhardtii shows lower expression than CHLI1, strains overexpressing CHLI2 were produced in the chli1 background. However, no complementation of the chli1 phenotype was observed. Silencing of CHLI2 in the wild-type background did not result in any changes in the accumulation of tetrapyrrole intermediates or of chlorophyll. The results suggest that, unlike in A. thaliana, changes in CHLI2 content observed in the present studies do not affect formation and activity of MgCh in C. reinhardtii.
authors.affiliation: Institute of Biology/Plant Physiology, Humboldt University, Philippstraße 13, D-10115 Berlin, Germany.
authors.forename: Pawel
authors.initials: P
authors.lastname: Brzezowski
authors.affiliation: Department of Plant and Microbial Biology, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720-3102, USA Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
authors.forename: Marina N
authors.initials: MN
authors.lastname: Sharifi
authors.affiliation: Department of Plant and Microbial Biology, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720-3102, USA Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Dent
authors.affiliation: Institute of Biology/Plant Physiology, Humboldt University, Philippstraße 13, D-10115 Berlin, Germany.
authors.forename: Marius K
authors.initials: MK
authors.lastname: Morhard
authors.affiliation: Department of Plant and Microbial Biology, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720-3102, USA Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
authors.forename: Krishna K
authors.initials: KK
authors.lastname: Niyogi
authors.affiliation: Institute of Biology/Plant Physiology, Humboldt University, Philippstraße 13, D-10115 Berlin, Germany bernhard.grimm@rz.hu-berlin.de.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Grimm
mesh_terms: D020418:Algal Proteins / Q000235:genetics* / Q000378:metabolism; D016825:Chlamydomonas reinhardtii / Q000235:genetics* / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D015536:Down-Regulation; D008190:Lyases / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D017422:Sequence Analysis, DNA; D015398:Signal Transduction; D045725:Tetrapyrroles / Q000378:metabolism
chemical_list: D020418:Algal Proteins; D045725:Tetrapyrroles; D002734:Chlorophyll; D008190:Lyases; C086788:magnesium chelatase
keywords: CHLI; Chlamydomonas; Mg chelatase; chloroplast retrograde signaling; protoporphyrin IX; tetrapyrrole biosynthesis pathway.
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