The ferredoxin-thioredoxin system of a green alga,Chlamydomonas reinhardtii

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Abstract
The components of the ferredoxin-thioredoxin (FT) system ofChlamydomonas reinhardtii have been purified and characterized. The system resembled that of higher plants in consisting of a ferredoxin-thioredoxin reductase (FTR) and two types of thioredoxin, a singlef and twom species,m1 andm2. TheChlamydomonas m andf thioredoxins were antigenically similar to their higher-plant counterparts, but not to one another. Them thioredoxins were recognized by antibodies to both higher-plantm and bacterial thioredoxins, whereas the thioredoxinf was not.Chlamydomonas thioredoxinf reacted, although weakly, with the antibody to spinach thioredoxinf. The algal thioredoxinf differed from thioredoxins studied previously in behaving as a basic protein on ion-exchange columns. Purification revealed that the algal thioredoxins had molecular masses (Mrs) typical of thioredoxins from other sources,m1 andm2 being 10700 andf 11 500.Chlamydomonas FTR had two dissimilar subunits, a feature common to all FTRs studied thus far. One, the 13-kDa (“similar”) subunit, resembled its counterpart from other sources in both size and antigenicity. The other, 10-kDa (“variable”) sub-unit was not recognized by antibodies to any FTR tested. When combined with spinach, (Spinacia oleracea L.) thylakoid membranes, the components of the FT system functioned in the light activation of the standard target enzymes from chloroplasts, corn (Zea mays L.) NADP-malate dehydrogenase (EC 1.1.1.82) and spinach fructose 1,6-bisphosphatase (EC 3.1.3.11) as well as the chloroplast-type fructose 1,6-bisphosphatase fromChlamydomonas. Activity was greatest if ferredoxin and other components of the FT system were fromChlamydomonas. The capacity of theChlamydomonas FT system to activate autologous FBPase indicates that light regulates the photosynthetic carbon metabolism of green algae as in other oxygenic photosynthetic organisms.