COORDINATION OF CHLOROPLASTIC METABOLISM IN N-LIMITED CHLAMYDOMONAS-REINHARDTII BY REDOX MODULATION .1. THE ACTIVATION OF PHOSPHORIBULOSEKINASE AND GLUCOSE-6-PHOSPHATE-DEHYDROGENASE IS RELATIVE TO THE PHOTOSYNTHETIC SUPPLY OF ELECTRONS

Extraction of Chlamydomonas reinhardtii CW-15 cells by rapid freezing and thawing demonstrates that the in vivo activity of the algal glucos e-6-phosphate dehydrogenase (G6PDH) is inhibited by the presence of li ght and activated in the dark, whereas phosphoribulosekinase (PRK) is light activated and inhibited in the dark. The effects of darkening ar e reversed by incubation with dithiothreitol (DTT) and mimicked by che mical oxidants, indicating that, as in higher plants, reduction via th e ferredoxin-thioredoxin system likely regulates these enzymes. The tw o enzymes varied in their sensitivity to reduction; the inclusion of 0 .5 mM DTT during extraction inhibited G6PDH, whereas PRK required trea tment with 40 mM DTT for 1 h to reach maximum activation. The activati on change for both enzymes was nearly complete within the 1st min afte r cells were transferred between light and dark, but the level of acti vation was relative to the incident light at low intensities; G6PDH ac tivity decreased with increasing light, whereas PRK became more active . The reductive inhibition of G6PDH saturated at very low light, where as PRK activation kinetics closely followed the increase in photosynth etic 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.