In the chlorophyte alga Chlamydomonas reinhardtii , expression of the nuclear gene NIA1 , encoding nitrate reductase, is regulated by light, but the signal transduction mechanism is poorly understood. Using inhibitors, mutants, and physiological manipulation, we searched for signals in the photosynthetic electron transport chain that potentially regulate NIA1 expression. In the NIA1 + wild-type clone CC-1692, nitrate reductase activity is strongly down-regulated when the reduction of plastoquinone is blocked by 3-(3'4'-dichlorophenyl)-1,1'-dimethyl urea (DCMU), but unaffected or stimulated when the oxidation of plastoquinol is inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Simultaneously, although DBMIB reduced NIA1 expression by?~30% over a 6-h period relative to the control, DCMU inhibited expression of the gene by over 80%. A cross between CC-1692 and a site-directed mutant, CC-3388 A251I , in which amino acid 251 in the PSII core protein, D1, was altered from alanine to isoleucine, thereby decreasing the binding affinity for Q B , produced a cell with markedly reduced expression of NIA1 . Our results indicate that expression of nitrate reductase is coupled to photosynthesis via a sensor related to the redox poise of the plastoquinone pool. When the pool is oxidized, carbon fixation is low and nitrate reductase is down-regulated; conversely, when the pool is reduced, carbon fixation is high and the gene and enzyme activity are up-regulated. These experimental observations suggest a model for the coupled light regulation of photosynthesis and nitrate assimilation.
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Document Type: Research Article
Dipartimento di Scienze del Mare, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy
Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08901-8521, USA
Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854-8066, USA
Publication date: 2005-11-01