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)

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.