The ferredoxin/thioredoxin system of enzyme regulation in a cyanobacterium

Abstract

Cell-free preparations of the cyanobacterium (bluegreen alga) Nostoc muscorum were assayed for thioredoxins and enzymes catalyzing the ferredoxin and NADP-linked reduction of thioredoxin. Nostoc was found to have two different thioredoxins: one of approximate molecular weight 16,000 (designated Nostoc thioredoxin f) that selectively activated chloroplast fructose 1,6-bisphosphatase, and another of approximate molecular weight 9,000 (designated Nostoc thioredoxin m) that selcetively activated chloroplast NADP-malate dehydrogenase. The two thioredoxins could be reduced either chemically with dithiothreitol or photochemically with ferredoxin and ferredoxin-thioredoxin reductase which, like the recently found regulatory iron-sulfur protein ferralterin, was present in Nostoc cells. Nostoc ferredoxin-thioredoxin reductase appeared to be similar to its chloroplast counterpart in enzyme specificity, molecular weight, and spectral properties. The Nostoc and spinach chloroplast ferredoxin-thioredoxin reductases as well as their thioredoxins, ferredoxins, and chlorophyll containing membranes were interchangeable in activating chloroplast fructose 1,6-bisphosphatase and NADP-malate dehydrogenase. There was no evidence for an NADP-linked thioredoxin reductase such as that of E. coli. The results are in accord with the conclusion that the cyanobacteria resemble higher plants in having a functional ferredoxin/thioredoxin system rather than an NADP/thioredoxin system typical of other bacteria.