Protein modulase and ferredoxin/thioredoxin reductase are soluble proteins that have been suggested to catalyze the light-dependent modulation of enzyme activity in the stromal compartment of the chloroplast. Protein modulase is active in vitro without additional ferredoxin and thioredoxin, whereas ferredoxin/thioredoxin reductase requires additional ferredoxin and thioredoxin. We hypothesize that protein modulase is a complex protein composed of ferredoxin/thioredoxin reductase, ferredoxin, and thioredoxin. In recenstituted chloroplast systems, antiserum directed against ferredoxin, at concentrations sufficient to inhibit the photoreduction of NADP, had no effect on light modulation. Antiserum directed against thioredoxin gave variable results: one batch of polyclonal antibodies inhibited light modulation, another was stimulatory, and another was without effect. These results suggest that the ferredoxin and thioredoxin active in light modulation are not free in solution. Furthermore, molecular sieve chromatography of stromal proteins results in the elution of four species that catalyze light modulation. Based on whether or not ferredoxin and/or thioredoxin must be added for activity, these four species have been tentatively identified as protein modulase, a complex of ferredoxin/thioredoxin reductase and ferredoxin, a complex of ferredoxin/thioredoxin reductase and thioredoxin, and ferredoxin/ thioredoxin reductase. That is, the four correspond to all the possible combinations of ferredoxin, ferredoxin/thioredoxin reductase, and thioredoxin. We suggest that buffer ionic strength affects the interactions among these proteins and in part determines the fate of the protein modulase complex in vitro.The activity of several chloroplastic enzymes is modulated during the dark/light transition. Three different systems (the 'light effect mediator,' the 'ferredoxin/thioredoxin,' and the 'ferralterin' systems) have been proposed to account for this phenomenon. One difference among these is the nature of the stromal factor(s) that putatively shuttle(s) electrons from PSI to the target enzymes (1,8,15).The activity of these stromal factors is assayed in reconstituted chloroplasts (created by mixing washed thylakoids with stromal extracts) by observing the change in activity of a target enzyme.Of the differences among the stromal factors that catalyze light 'Supported by National Science Foundation Grant DBM 84 17081. modulation, the most significant is whether or not thioredoxin and ferredoxin need be included in the assay mixture. Protein modulase and ferralterin, the stromal factors of the LEM' and ferralterin systems, are active in the absence of added Td and Fd. Ferredoxin/thioredoxin reductase activity requires exogenous Td and Fd. We hypothesize that PM, and likely ferralterin, is a complex protein composed of FTR, Fd, and Td. The soluble components of the proposed systems, then, would simply differ in the extent to which this complex has broken down. The results we report here are consistent with this hypothe...
