COORDINATION OF CHLOROPLASTIC METABOLISM IN N-LIMITED CHLAMYDOMONAS-REINHARDTII BY REDOX MODULATION .2. REDOX MODULATION ACTIVATES THE OXIDATIVE PENTOSE-PHOSPHATE PATHWAY DURING PHOTOSYNTHETIC NITRATE ASSIMILATION

The onset of photosynthetic NO3- assimilation in N-limited Chlamydomon as reinhardtii increased the initial extractable activity of the gluco se-6-phosphate dehydrogenase (G6PDH), the key regulatory step of the o xidative pentose phosphate pathway. The total activated enzyme activit y did not change upon NO3- resupply. The higher activity, therefore, r epresents 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- assimila tion. A 2-fold stimulation of O-2 evolution and a 70% decrease in the rate of photosynthetic CO2 assimilation accompanied the enzyme activit y changes. There was an immediate drop in the NADPH and an increase in NADP upon addition of NO3-, whereas NH4+ caused only minor fluctuatio ns in these pools. The response of C. reinhardtii to NO3- indicates th at the oxidative pentose phosphate pathway was activated to oxidize ca rbon upon the onset of NO3- assimilation, whereas reduction of carbon via the reductive pentose phosphate pathway was inhibited. This demons trates a possible role for the Fd-thioredoxin system in coordinating e nzyme activity in response to the metabolic demands for reducing power and carbon during NO3- assimilation.