A Large Chloroplast Thioredoxin/Found in Green Algae

Unicellular green algae differ from plant leaves in their thioredoxin profile. Besides several thioredoxins of regular size (Mr = 12,000), the heat-stable protein fraction of extracts from Scenedesmus obliquus cells contains a large protein of molecular weight M r — 28,000 which is designated thioredoxin /o n the basis of typical properties, in particular by its capacity to stimulate spinach chloroplast fructose-bis-phosphatase and, to lower degree, E. coli ribonucleotide reduc­ tase. The new thioredoxin was purified to apparent homogeneity by chromatography on DEAE cellulose, Sephadex G-50. CM cellulose, and Blue Sepharose. When tested in homologous en­ zyme systems, reduced thioredoxin / strongly activated algal fructose-bis-phosphatase, but was inactive towards the cytoplasmic algal ribonucleotide reductase; NADP malate dehydrogenase was also stimulated. The protein is missing in extracts from a chloroplast-free mutant strain, C-2A', but appears together with other chloroplast components upon illumination. Protein / is therefore the main chloroplast thioredoxin of the green algae, probably corresponding to the smaller leaf chloroplast thioredoxins / and m combined. Algal thioredoxin / appears closely related, however, to the large thioredoxin found in a cyanobacterium, Anabaena sp.