II. Redox Modulation Activates the Oxidative Pentose Phosphate Pathway during Photosynthetic Nitrate Assimilation

l h e onset of photosynthetic NO3assimilation in N-limited Chlamydomonas reinhardtii increased the initial extractable activity of the glucose-6-phosphate dehydrogenase (C6PDH), the key regulatory step of the oxidative pentose phosphate pathway. l h e total activated enzyme activity did not change upon NO3resupply. l h e higher activity, therefore, represents activation of existing enzyme. No activation occurred during NH.+ assimilation. Incubation of extracts with D l l reversed the NO3stimulation of C6PDH activity, indicating that the activation involved redox modulation of G6PDH. Phosphoribulosekinase, an enzyme activated by thioredoxin reduction, was inhibited at the onset of NO3assimilation. A 2-fold stimulation of O2 evolution and a 70% decrease in the rate of photosynthetic C 0 2 assimilation accompanied the enzyme activity changes. There was an immediate drop in the NADPH and an increase in NADP upon addition of NO3-, whereas NH,+ caused only minor fluctuations in these pools. l h e response of C. reinhardtii to NO3indicates that the oxidative pentose phosphate pathway was activated to oxidize carbon upon the onset of NO3assimilation, whereas reduction of carbon via the reductive pentose phosphate pathway was inhibited. lhis 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 NO3assimilation.