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Sinha N, Eirich J, Finkemeier I, Grimm B. Glutamate 1-semialdehyde aminotransferase is connected to GluTR by GluTR-binding protein and contributes to the rate-limiting step of 5-aminolevulinic acid synthesis. THE PLANT CELL 2022; 34:4623-4640. [PMID: 35972388 PMCID: PMC9614494 DOI: 10.1093/plcell/koac237] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/13/2022] [Accepted: 07/28/2022] [Indexed: 06/01/2023]
Abstract
Tetrapyrroles play fundamental roles in crucial processes including photosynthesis, respiration, and catalysis. In plants, 5-aminolevulinic acid (ALA) is the common precursor of tetrapyrroles. ALA is synthesized from activated glutamate by the enzymes glutamyl-tRNA reductase (GluTR) and glutamate-1-semialdehyde aminotransferase (GSAAT). ALA synthesis is recognized as the rate-limiting step in this pathway. We aimed to explore the contribution of GSAAT to the control of ALA synthesis and the formation of a protein complex with GluTR. In Arabidopsis thaliana, two genes encode GSAAT isoforms: GSA1 and GSA2. A comparison of two GSA knockout mutants with the wild-type revealed the correlation of reduced GSAAT activity and ALA-synthesizing capacity in leaves with lower chlorophyll content. Growth and green pigmentation were more severely impaired in gsa2 than in gsa1, indicating the predominant role of GSAAT2 in ALA synthesis. Interestingly, GluTR accumulated to higher levels in gsa2 than in the wild-type and was mainly associated with the plastid membrane. We propose that the GSAAT content modulates the amount of soluble GluTR available for ALA synthesis. Several different biochemical approaches revealed the GSAAT-GluTR interaction through the assistance of GluTR-binding protein (GBP). A modeled structure of the tripartite protein complex indicated that GBP mediates the stable association of GluTR and GSAAT for adequate ALA synthesis.
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Affiliation(s)
- Neha Sinha
- Institute of Biology/Plant Physiology, Humboldt-University Berlin, 10115 Berlin, Germany
| | - Jürgen Eirich
- Institute for Plant Biology and Biotechnology, Plant Physiology, Westfälische Wilhelms-Universität, 48149 Muenster, Germany
| | - Iris Finkemeier
- Institute for Plant Biology and Biotechnology, Plant Physiology, Westfälische Wilhelms-Universität, 48149 Muenster, Germany
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Wang Q, Zhu B, Chen C, Yuan Z, Guo J, Yang X, Wang S, Lv Y, Liu Q, Yang B, Sun C, Wang P, Deng X. A Single Nucleotide Substitution of GSAM Gene Causes Massive Accumulation of Glutamate 1-Semialdehyde and Yellow Leaf Phenotype in Rice. RICE (NEW YORK, N.Y.) 2021; 14:50. [PMID: 34089406 PMCID: PMC8179877 DOI: 10.1186/s12284-021-00492-x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2021] [Accepted: 05/12/2021] [Indexed: 05/04/2023]
Abstract
BACKGROUND Tetrapyrroles play indispensable roles in various biological processes. In higher plants, glutamate 1-semialdehyde 2,1-aminomutase (GSAM) converts glutamate 1-semialdehyde (GSA) to 5-aminolevulinic acid (ALA), which is the rate-limiting step of tetrapyrrole biosynthesis. Up to now, GSAM genes have been successively identified from many species. Besides, it was found that GSAM could form a dimeric protein with itself by x-ray crystallography. However, no mutant of GSAM has been identified in monocotyledonous plants, and no experiment on interaction of GSAM protein with itself has been reported so far. RESULT We isolated a yellow leaf mutant, ys53, in rice (Oryza sativa). The mutant showed decreased photosynthetic pigment contents, suppressed chloroplast development, and reduced photosynthetic capacity. In consequence, its major agronomic traits were significantly affected. Map-based cloning revealed that the candidate gene was LOC_Os08g41990 encoding GSAM protein. In ys53 mutant, a single nucleotide substitution in this gene caused an amino acid change in the encoded protein, so its ALA-synthesis ability was significantly reduced and GSA was massively accumulated. Complementation assays suggested the mutant phenotype of ys53 could be rescued by introducing wild-type OsGSAM gene, confirming that the point mutation in OsGSAM is the cause of the mutant phenotype. OsGSAM is mainly expressed in green tissues, and its encoded protein is localized to chloroplast. qRT-PCR analysis indicated that the mutation of OsGSAM not only affected the expressions of tetrapyrrole biosynthetic genes, but also influenced those of photosynthetic genes in rice. In addition, the yeast two-hybrid experiment showed that OsGSAM protein could interact with itself, which could largely depend on the two specific regions containing the 81th-160th and the 321th-400th amino acid residues at its N- and C-terminals, respectively. CONCLUSIONS We successfully characterized rice GSAM gene by a yellow leaf mutant and map-based cloning approach. Meanwhile, we verified that OsGSAM protein could interact with itself mainly by means of the two specific regions of amino acid residues at its N- and C-terminals, respectively.
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Affiliation(s)
- Qian Wang
- State Key Laboratory of Crop Gene Exploration and Utilization in Southwest China, Sichuan Agricultural University, Chengdu, 611130, China
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Baiyang Zhu
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Congping Chen
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Zhaodi Yuan
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Jia Guo
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Xiaorong Yang
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - San Wang
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Yan Lv
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Qingsong Liu
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Bin Yang
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Changhui Sun
- State Key Laboratory of Crop Gene Exploration and Utilization in Southwest China, Sichuan Agricultural University, Chengdu, 611130, China
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China
| | - Pingrong Wang
- State Key Laboratory of Crop Gene Exploration and Utilization in Southwest China, Sichuan Agricultural University, Chengdu, 611130, China.
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China.
| | - Xiaojian Deng
- State Key Laboratory of Crop Gene Exploration and Utilization in Southwest China, Sichuan Agricultural University, Chengdu, 611130, China.
- Rice Research Institute, Sichuan Agricultural University, Chengdu, 611130, China.
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Hellmann H, Goyer A, Navarre DA. Antioxidants in Potatoes: A Functional View on One of the Major Food Crops Worldwide. Molecules 2021; 26:2446. [PMID: 33922183 PMCID: PMC8122721 DOI: 10.3390/molecules26092446] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2021] [Revised: 04/07/2021] [Accepted: 04/19/2021] [Indexed: 12/14/2022] Open
Abstract
With a growing world population, accelerating climate changes, and limited arable land, it is critical to focus on plant-based resources for sustainable food production. In addition, plants are a cornucopia for secondary metabolites, of which many have robust antioxidative capacities and are beneficial for human health. Potato is one of the major food crops worldwide, and is recognized by the United Nations as an excellent food source for an increasing world population. Potato tubers are rich in a plethora of antioxidants with an array of health-promoting effects. This review article provides a detailed overview about the biosynthesis, chemical and health-promoting properties of the most abundant antioxidants in potato tubers, including several vitamins, carotenoids and phenylpropanoids. The dietary contribution of diverse commercial and primitive cultivars are detailed and document that potato contributes much more than just complex carbohydrates to the diet. Finally, the review provides insights into the current and future potential of potato-based systems as tools and resources for healthy and sustainable food production.
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Affiliation(s)
- Hanjo Hellmann
- School of Biological Sciences, Washington State University, Pullman, WA 99164, USA
| | - Aymeric Goyer
- Hermiston Agricultural Research and Extension Center, Department of Botany and Plant Pathology, Oregon State University, Hermiston, OR 97838, USA;
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Del Mondo A, Smerilli A, Sané E, Sansone C, Brunet C. Challenging microalgal vitamins for human health. Microb Cell Fact 2020; 19:201. [PMID: 33138823 PMCID: PMC7607653 DOI: 10.1186/s12934-020-01459-1] [Citation(s) in RCA: 52] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/24/2020] [Accepted: 10/17/2020] [Indexed: 01/09/2023] Open
Abstract
BACKGROUND Vitamins' deficiency in humans is an important threat worldwide and requires solutions. In the concept of natural biofactory for bioactive compounds production, microalgae represent one of the most promising targets filling many biotechnological applications, and allowing the development of an eco-sustainable production of natural bioactive metabolites. Vitamins are probably one of the cutting edges of microalgal diversity compounds. MAIN TEXT Microalgae can usefully provide many of the required vitamins in humans, more than terrestrial plants, for instance. Indeed, vitamins D and K, little present in many plants or fruits, are instead available from microalgae. The same occurs for some vitamins B (B12, B9, B6), while the other vitamins (A, C, D, E) are also provided by microalgae. This large panel of vitamins diversity in microalgal cells represents an exploitable platform in order to use them as natural vitamins' producers for human consumption. This study aims to provide an integrative overview on vitamins content in the microalgal realm, and discuss on the great potential of microalgae as sources of different forms of vitamins to be included as functional ingredients in food or nutraceuticals for the human health. We report on the biological roles of vitamins in microalgae, the current knowledge on their modulation by environmental or biological forcing and on the biological activity of the different vitamins in human metabolism and health protection. CONCLUSION Finally, we critically discuss the challenges for promoting microalgae as a relevant source of vitamins, further enhancing the interests of microalgal "biofactory" for biotechnological applications, such as in nutraceuticals or cosmeceuticals.
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Affiliation(s)
- Angelo Del Mondo
- Stazione Zoologica Anton Dohrn, Istituto Nazionale Di Biologia, Ecologia e Biotecnologie marine, Villa Comunale, 80121, Napoli, Italy
| | - Arianna Smerilli
- Stazione Zoologica Anton Dohrn, Istituto Nazionale Di Biologia, Ecologia e Biotecnologie marine, Villa Comunale, 80121, Napoli, Italy
| | - Elisabet Sané
- Stazione Zoologica Anton Dohrn, Istituto Nazionale Di Biologia, Ecologia e Biotecnologie marine, Villa Comunale, 80121, Napoli, Italy
| | - Clementina Sansone
- Stazione Zoologica Anton Dohrn, Istituto Nazionale Di Biologia, Ecologia e Biotecnologie marine, Villa Comunale, 80121, Napoli, Italy.
| | - Christophe Brunet
- Stazione Zoologica Anton Dohrn, Istituto Nazionale Di Biologia, Ecologia e Biotecnologie marine, Villa Comunale, 80121, Napoli, Italy
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Bagheri S, Pazoki F, Esfandiary N, Fadaei MM, Heydari A. Synthesis and characterization of Pd(II)–vitamin B
6
complex supported on magnetic nanoparticle as an efficient and recyclable catalyst system for C–N cross coupling of amides in deep eutectic solvents. Appl Organomet Chem 2020. [DOI: 10.1002/aoc.5723] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Affiliation(s)
- Sepideh Bagheri
- Department of Chemistry, Faculty of Sciences Tarbiat Modares University Tehran 14117‐13116 Iran
| | - Farzane Pazoki
- Department of Chemistry, Faculty of Sciences Tarbiat Modares University Tehran 14117‐13116 Iran
| | - Naghmeh Esfandiary
- Department of Chemistry, Faculty of Sciences Tarbiat Modares University Tehran 14117‐13116 Iran
| | - Mohammad Mahdi Fadaei
- Department of Chemistry, Faculty of Sciences Tarbiat Modares University Tehran 14117‐13116 Iran
| | - Akbar Heydari
- Department of Chemistry, Faculty of Sciences Tarbiat Modares University Tehran 14117‐13116 Iran
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Parra M, Stahl S, Hellmann H. Vitamin B₆ and Its Role in Cell Metabolism and Physiology. Cells 2018; 7:cells7070084. [PMID: 30037155 PMCID: PMC6071262 DOI: 10.3390/cells7070084] [Citation(s) in RCA: 196] [Impact Index Per Article: 32.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2018] [Revised: 07/18/2018] [Accepted: 07/20/2018] [Indexed: 12/11/2022] Open
Abstract
Vitamin B6 is one of the most central molecules in cells of living organisms. It is a critical co-factor for a diverse range of biochemical reactions that regulate basic cellular metabolism, which impact overall physiology. In the last several years, major progress has been accomplished on various aspects of vitamin B6 biology. Consequently, this review goes beyond the classical role of vitamin B6 as a cofactor to highlight new structural and regulatory information that further defines how the vitamin is synthesized and controlled in the cell. We also discuss broader applications of the vitamin related to human health, pathogen resistance, and abiotic stress tolerance. Overall, the information assembled shall provide helpful insight on top of what is currently known about the vitamin, along with addressing currently open questions in the field to highlight possible approaches vitamin B6 research may take in the future.
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Affiliation(s)
- Marcelina Parra
- Hellmann Lab, School of Biological Sciences, College of Liberal Arts and Sciences, Washington State University, Pullman, 99164-6234 WA, USA.
| | - Seth Stahl
- Hellmann Lab, School of Biological Sciences, College of Liberal Arts and Sciences, Washington State University, Pullman, 99164-6234 WA, USA.
| | - Hanjo Hellmann
- Hellmann Lab, School of Biological Sciences, College of Liberal Arts and Sciences, Washington State University, Pullman, 99164-6234 WA, USA.
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Rueschhoff EE, Gillikin JW, Sederoff HW, Daub ME. The SOS4 pyridoxal kinase is required for maintenance of vitamin B6-mediated processes in chloroplasts. PLANT PHYSIOLOGY AND BIOCHEMISTRY : PPB 2013; 63:281-91. [PMID: 23321022 DOI: 10.1016/j.plaphy.2012.12.003] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/12/2012] [Accepted: 12/14/2012] [Indexed: 05/01/2023]
Abstract
Vitamin B(6) (pyridoxal 5'-phosphate and its vitamers) is an important cofactor in numerous enzymatic reactions. In spite of its importance, the consequences of altering vitamin B(6) content on plant growth and development are not well understood. This study compares two mutants for vitamin B(6)-metabolizing enzymes in Arabidopsis thaliana: a pdx1.3 mutant in the de novo synthesis pathway and a salvage pathway sos4 mutant that accumulates more vitamin B(6). We show that despite a difference in total B(6) content in leaf tissue, both mutants share similar phenotypes, including chlorosis, decreased size, altered chloroplast ultrastructure, and root sensitivity to sucrose. Assay of B(6) vitamer content from isolated chloroplasts showed that, despite differing B(6) vitamer content in whole leaf tissue, both mutants share a common deficiency in total and phosphorylated vitamers in chloroplasts. One of the splice variants of the SOS4 proteins was shown to be located in the chloroplast. Our data indicate that some of the phenotypic consequences shared between the pdx1.3 and sos4 mutants are due to B(6) deficiency in chloroplasts, and show that SOS4 is required for maintenance of phosphorylated B(6) vitamer concentrations in chloroplasts. Further, our data are consistent with a diffusion model for transport of vitamin B(6) into chloroplasts.
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Affiliation(s)
- Elizabeth E Rueschhoff
- Department of Plant Biology, North Carolina State University, Raleigh, NC 27695-7612, USA.
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Herrero S, González E, Gillikin JW, Vélëz H, Daub ME. Identification and characterization of a pyridoxal reductase involved in the vitamin B6 salvage pathway in Arabidopsis. PLANT MOLECULAR BIOLOGY 2011; 76:157-69. [PMID: 21533842 DOI: 10.1007/s11103-011-9777-x] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/08/2010] [Accepted: 04/08/2011] [Indexed: 05/22/2023]
Abstract
Vitamin B6 (pyridoxal phosphate) is an essential cofactor in enzymatic reactions involved in numerous cellular processes and also plays a role in oxidative stress responses. In plants, the pathway for de novo synthesis of pyridoxal phosphate has been well characterized, however only two enzymes, pyridoxal (pyridoxine, pyridoxamine) kinase (SOS4) and pyridoxamine (pyridoxine) 5' phosphate oxidase (PDX3), have been identified in the salvage pathway that interconverts between the six vitamin B6 vitamers. A putative pyridoxal reductase (PLR1) was identified in Arabidopsis based on sequence homology with the protein in yeast. Cloning and expression of the AtPLR1 coding region in a yeast mutant deficient for pyridoxal reductase confirmed that the enzyme catalyzes the NADPH-mediated reduction of pyridoxal to pyridoxine. Two Arabidopsis T-DNA insertion mutant lines with insertions in the promoter sequences of AtPLR1 were established and characterized. Quantitative RT-PCR analysis of the plr1 mutants showed little change in expression of the vitamin B6 de novo pathway genes, but significant increases in expression of the known salvage pathway genes, PDX3 and SOS4. In addition, AtPLR1 was also upregulated in pdx3 and sos4 mutants. Analysis of vitamer levels by HPLC showed that both plr1 mutants had lower levels of total vitamin B6, with significantly decreased levels of pyridoxal, pyridoxal 5'-phosphate, pyridoxamine, and pyridoxamine 5'-phosphate. By contrast, there was no consistent significant change in pyridoxine and pyridoxine 5'-phosphate levels. The plr1 mutants had normal root growth, but were significantly smaller than wild type plants. When assayed for abiotic stress resistance, plr1 mutants did not differ from wild type in their response to chilling and high light, but showed greater inhibition when grown on NaCl or mannitol, suggesting a role in osmotic stress resistance. This is the first report of a pyridoxal reductase in the vitamin B6 salvage pathway in plants.
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Affiliation(s)
- Sonia Herrero
- Department of Plant Biology, North Carolina State University, Raleigh, NC 27695-7612, USA
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Lv X, Fan J, Ge H, Gao Y, Zhang X, Teng M, Niu L. Cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of the glutamate-1-semialdehyde aminotransferase from Bacillus subtilis. Acta Crystallogr Sect F Struct Biol Cryst Commun 2006; 62:483-5. [PMID: 16682782 PMCID: PMC2219978 DOI: 10.1107/s1744309106013121] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2006] [Accepted: 04/11/2006] [Indexed: 11/10/2022]
Abstract
5-Aminolevulinic acid (ALA) is the first committed universal precursor in the tetrapyrrole-biosynthesis pathway. Plants, algae and many other bacteria synthesize ALA from glutamate by a C5 pathway in which the carbon skeleton of glutamate is converted into ALA by a series of enzymes. Glutamate-1-semialdehyde aminotransferase (GSAT) is the last enzyme in this pathway. The gene that codes for GSAT was amplified from the cDNA library of Bacillus subtilis and overexpressed in Escherichia coli strain BL21(DE3). The protein was purified and crystallized. Well diffracting single crystals were obtained by the hanging-drop vapour-diffusion method. Preliminary X-ray diffraction studies yielded excellent diffraction data to a resolution of 2.0 angstroms.
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Affiliation(s)
- Xinhuai Lv
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
| | - Jun Fan
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
| | - Honghua Ge
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
| | - Yongxiang Gao
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
| | - Xiao Zhang
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
| | - Maikun Teng
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Correspondence e-mail: ,
| | - Liwen Niu
- Hefei National Laboratory for Physical Sciences at Microscale and School of Life Sciences, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Key Laboratory of Structural Biology, Chinese Academy of Sciences, 96 Jinzhai Road, Hefei, Anhui 230027, People’s Republic of China
- Correspondence e-mail: ,
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Nogaj LA, Srivastava A, van Lis R, Beale SI. Cellular levels of glutamyl-tRNA reductase and glutamate-1-semialdehyde aminotransferase do not control chlorophyll synthesis in Chlamydomonas reinhardtii. PLANT PHYSIOLOGY 2005; 139:389-96. [PMID: 16126849 PMCID: PMC1203387 DOI: 10.1104/pp.105.067009] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/04/2023]
Abstract
5-Aminolevulinic acid (ALA) is the first committed universal precursor in the tetrapyrrole biosynthesis pathway. In plants, algae, and most bacteria, ALA is generated from glutamate. First, glutamyl-tRNA synthetase activates glutamate by ligating it to tRNA(Glu). Activated glutamate is then converted to glutamate 1-semialdehyde (GSA) by glutamyl-tRNA reductase (GTR). Finally, GSA is rearranged to ALA by GSA aminotransferase (GSAT). In the unicellular green alga Chlamydomonas reinhardtii, GTR and GSAT were found in the chloroplasts and were not detected in the mitochondria by immunoblotting. The levels of both proteins (assayed by immunoblotting) and their mRNAs (assayed by RNA blotting) were approximately equally abundant in cells growing in continuous dark or continuous light (fluorescent tubes, 80 micromol photons s(-1) m(-2)), consistent with the ability of the cells to form chlorophyll under both conditions. In cells synchronized to a 12-h-light/12-h-dark cycle, chlorophyll accumulated only during the light phase. However, GTR and GSAT were present at all phases of the cycle. The GTR mRNA level increased in the light and peaked about 2-fold at 2 h into the light phase, and GTR protein levels also increased and peaked 2-fold at 4 to 6 h into the light phase. In contrast, although the GSAT mRNA level increased severalfold at 2 h into the light phase, the level of GSAT protein remained approximately constant in the light and dark phases. Under all growth conditions, the cells contained significantly more GSAT than GTR on a molar basis. Our results indicate that the rate of chlorophyll synthesis in C. reinhardtii is not directly controlled by the expression levels of the mRNAs for GTR or GSAT, or by the cellular abundance of these enzyme proteins.
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Affiliation(s)
- Luiza A Nogaj
- Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912, USA
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