Light-regulated translation of chloroplast messenger RNAs (mRNAs) requires transacting factors that interact with the 5’ untranslated region (UTR) of these mRNAs, Chloroplast polyadenylate-binding protein (cPABP) specifically binds to the 5’-UTR of the psbA mRNA and is essential for translation of this mRNA, A protein disulfide isomerase that is localized to the chloroplast and copurifies with cPABP was shown to modulate the binding of cPABP to the 5’-UTR of the psbA mRNA by reversibly changing the redox status of cPABP through redox potential or adenosine 5’-diphosphate-dependent phosphorylation. This mechanism allows for a simple reversible switch regulating gene expression in the chloroplast.