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Photorespiratory ammonium assimilation in chloroplasts of Chlamydomonas reinhardtii
Author(s) -
Ramazanov Ziyadin,
Cárdenas Jacobo
Publication year - 1994
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1994.tb02979.x
Subject(s) - chlamydomonas reinhardtii , ammonium , glutamine synthetase , carbonic anhydrase , chlamydomonas , biochemistry , chloroplast , photorespiration , excretion , biology , mutant , extracellular , chemistry , glutamine , enzyme , amino acid , gene , organic chemistry
The effect of CO 2 concentration on the rate of photorespiratory ammonium excretion and on glutamine synthetase (GS) and carbonic anhydrase (CA) isoenzymes activities has been studied in Chlamydomonas reinhardtii cw‐15 mutant (lacking cell wall) and in the high CO 2 ‐requiring double mutant cia‐3/cw‐15 (lacking cell wall and chloroplastic carbonic anhydrase). In cw‐15 cells, both the extracellular (CA ext ) and chloroplastic (CA chl ) CA activities increased after transferring cells from media bubbled with 5% CO 2 in air (v/v, high‐C i cells) to 0.03% CO 2 (low‐C i cells), whereas in cia‐3/cw‐15 cells only the CA ext was induced after adaptation to low‐C i conditions and the CA chl activity was negligible. During adaptation to low‐C i conditions in the presence of 1 m M of l‐methionine‐D,L‐sulfoximine (MSX), a specific inhibitor of GS activity, both mutant strains excreted photorespiratory ammonium into nitrogen free medium. In addition, the ammonium excretion rate by cw‐15 in the presence of MSX was lower in cells grown and kept at 5% CO 2 than in high‐C i cells adapted to 0.03% CO 2 . The double mutant cia‐3/cw‐15 excreted photorespiratory ammonium at a higher rate than did cw‐15 . Total GS activity (GS‐1 plus GS‐2) increased during adaptation to 0.03% CO 2 in both strains of C. reinhardtii . However, only the activity GS‐2, which is located in the chloroplast, increased during the adaptation to low CO 2 , whereas the cytosolic GS‐1 levels remained similar in high and low‐C i cells. We conclude that: (1) cia‐3/cw‐15 cells lack chloroplastic CA activity; (2) in C. reinhardtii photorespiratory ammonium is refixed in the chloroplasts through the GS‐2/GOGAT cycle; and (3) chloroplastic GS‐2 concentration changes in response to the variation of environmental CO 2 concentration.