PLANT PHYSIOLOGY , Vol 105, Issue 4 1043-1048, 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 (II. Redox Modulation Activates the Oxidative Pentose Phosphate Pathway during Photosynthetic Nitrate Assimilation)
H. C. Huppe, T. J. Farr and D. H. Turpin
Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6
The onset of photosynthetic NO3- assimilation in N-limited Chlamydomonas
reinhardtii increased the initial extractable activity of the
glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory step of the
oxidative pentose phosphate pathway. The total activated enzyme activity
did not change upon NO3- resupply. The higher activity, therefore,
represents activation of existing enzyme. No activation occurred during
NH4+ assimilation. Incubation of extracts with DTT reversed the NO3-
stimulation of G6PDH activity, indicating that the activation involved
redox modulation of G6PDH. Phosphoribulosekinase, an enzyme activated by
thioredoxin reduction, was inhibited at the onset of NO3- assimilation. A
2-fold stimulation of O2 evolution and a 70% decrease in the rate of
photosynthetic CO2 assimilation accompanied the enzyme activity changes.
There was an immediate drop in the NADPH and an increase in NADP upon
addition of NO3-, whereas NH4+ caused only minor fluctuations in these
pools. The response of C. reinhardtii to NO3- indicates that the oxidative
pentose phosphate pathway was activated to oxidize carbon upon the onset of
NO3- assimilation, whereas reduction of carbon via the reductive pentose
phosphate pathway was inhibited. This demonstrates a possible role for the
Fd-thioredoxin system in coordinating enzyme activity in response to the
metabolic demands for reducing power and carbon during NO3- assimilation.
