Thioredoxin as a Modulator of Glucose - 6 - phosphate Dehydrogenase in a N , - Fixing Cyanobacterium

Glucose-6-phosphate dehydrogenase (G6PDH) is a key enzyme involved in fixed carbon dissimilation in photosynthetic microorganisms ; in heterocystous cyanobacteria it may also be implicated in the supply of reductant to nitrogenase. In crude cell-free extracts of the N,-fixing cyanobacterium Anabaena variabilis G6PDH activity was reversibly deactivated by the thiol agent dithiothreitol in the presence of a low molecular weight protein (12000 mol. wt). Glucose 6-phosphate reversed deactivation when added at high concentration, or prevented deactivation if added with the thiol. NADP+, which, like glucose 6-phosphate, is a G6PDH substrate, also deactivated the enzyme; deactivation was reversed or prevented by adding glucose 6-phosphate or glutamine. Purified thioredoxin from Anabaena cyfindrica, at very low concentrations (2 nM), deactivated purified G6PDH in a manner identical to that observed when crude extracts were used in the presence of dithiothreitol. Glutathione did not affect the enzyme. Cyanobacteria are photosynthetic prokaryotes which use water as source of reductant and fix C 0 2 via the Calvin cycle with ribulose-l,5-bisphosphate carboxylase serving as the key carboxylating enzyme, as in higher plants. Fixed carbon dissimilation is mainly via the oxidative pentose phosphate pathway (Pelroy & Bassham, 1972) with glucose-&phosphate dehydrogenase (G6PDH) being the first enzyme involved in hexose dissimilation via this route (see Stewart, 1980; Smith, 1982). There is also evidence that it may be involved in electron transfer to nitrogenase in cyanobacteria providing NADPH, from the dissimilation of fixed carbon, which transfers electrons to ferredoxin via ferredoxin-NADP + oxidoreductase and hence to nitrogenase (Apte et a/., 1978). This enzyme occupies a key branch point of carbon metabolism in cyanobacteria and thus its regulation is of particular importance. NADPH have all been shown to regulate G6PDH and there is evidence that it may be redox-modulated (Czeke et a/., 1981). However, the overall regulation of the enzyme is unclear (Schaeffer & Stanier, 1978). Here we provide evidence that thioredoxin may be involved in the modulation of the G6PDH of heterocystous cyanobacteria and consider ways in which this may be of importance. Organism and culture conditions. Anabaena i~arzahilis Kutz. ATCC 29413 was grown in axenic culture in one-eighth strength Allen & Arnon's medium (Allen & Arnon, 1955), without combined nitrogen unless stated otherwise, at 25 "C and at a photon flux density incident at the surface of the vessel of 70 pmol m-? s-I , in 10 1 batch culture or in continuous culture at a dilution rate of 0.03 …