Nitrate assimilation in Chlamydomonas as a model system to other organisms

Emilio Fernández1, Manuel Tejada-Jiménez1, Antonio Camargo1, Angel Llamas1, Amaury de Montaigu1, Emanuel Sanz-Luque1, Jose J. Higuera1, David González-Ballester1, Rogene A. Schnell2, Paul A. Lefebvre2, and Aurora Galván1

1Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Córdoba. Campus de Rabanales, Edif. Severo Ochoa, 14071-Córdoba, Spain; 2Department of Plant Biology, University of Minnesota, 250 Biological Sciences Center, 1445 Gortner Ave., St. Paul, MN 55108, USA

Nitrate assimilation is a fundamental pathway for incorporating the macroelement N by photosynthetic organisms like crop plants. The economic importance of nitrogen for crop productivity and the ecological side effects derived from the massive use of N fertilizers move research attention to understand better the nitrate pathway. This route is mechanistically simple, two transport and two reduction steps to produce ammonium in the chloroplast that is incorporated into carbon skeletons by the GS/GOGAT cycle, but its regulation is complex. Functional genomics strategies in Chlamydomonas showed this complexity and helped to know fundamental regulatory steps. An ordered mutant library, covering most of the algal genome allowed us to isolate more than a hundred of tagged-mutants defective in signalling. Mutants unable to sense the positive signal of nitrate showed NIT2 as the master gene for nitrate assimilation. NIT2 has GAF, leucine zipper, and RWP-RK domains typical of transcription factors. Intracellular nitrate is needed not for expression but for functionality of NIT2 on positive transcriptional regulation of the Chlamydomonas NIA1 gene. Mutants over sensing nitrate allowed identifying most genes for molybdenum cofactor (MoCo) biosynthesis. Molybdenum is a micronutrient essential for nitrate assimilation. A high affinity molybdate transporter, MOT1, has been identified and, a MoCo-carrier protein, MCP1, studied in detail.

e-mail address of presenting author: bb1feree@uco.es