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Studies on redox signaling and light regulation of chloroplast enzymes have highlighted the importance of the ferredoxin–thioredoxin thiol–disulfide interchange cascade. Recent research has focused on the intramolecular mechanism by which the reduction status of a chloroplast enzyme affects its catalytic properties, and site-directed mutagenesis has been used to identify the regulatory cysteines involved. For some of the thiol-regulated enzymes, structure–function studies have revealed that the complex conformational changes that occur might be associated with disulfide isomerization and auto-inhibition. Transgenic approaches indicate that this regulation constitutes a rapid means to adjust enzyme activity to metabolic needs.