PLANT PHYSIOLOGY , Vol 105, Issue 4 1037-1042, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Coordination of Chloroplastic Metabolism in N-Limited Chlamydomonas reinhardtii by Redox Modulation (I. The Activation of Phosphoribulosekinase and Glucose-6-Phosphate Dehydrogenase Is Relative to the Photosynthetic Supply of Electrons)
T. J. Farr, H. C. Huppe and D. H. Turpin
Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
Extraction of Chlamydomonas reinhardtii CW-15 cells by rapid freezing and
thawing demonstrates that the in vivo activity of the algal
glucose-6-phosphate dehydrogenase (G6PDH) is inhibited by the presence of
light and activated in the dark, whereas phosphoribulosekinase (PRK) is
light activated and inhibited in the dark. The effects of darkening are
reversed by incubation with dithiothreitol (DTT) and mimicked by chemical
oxidants, indicating that, as in higher plants, reduction via the
ferredoxin-thioredoxin system likely regulates these enzymes. The two
enzymes varied in their sensitivity to reduction; the inclusion of 0.5 mM
DTT during extraction inhibited G6PDH, whereas PRK required treatment with
40 mM DTT for 1 h to reach maximum activation. The activation change for
both enzymes was nearly complete within the 1st min after cells were
transferred between light and dark, but the level of activation was
relative to the incident light at low intensities; G6PDH activity decreased
with increasing light, whereas PRK became more active. The reductive
inhibition of G6PDH saturated at very low light, whereas PRK activation
kinetics closely followed the increase in photosynthetic oxygen evolution.
These results indicate that light-driven redox modulation of G6PDH and PRK
is more than an on/off switch, but acts to optimize the reduction and
oxidation of carbon in the chloroplast in accordance with the supply of
electrons.
