Plastidic redox switches

Ferredoxins (Fds) are small iron-sulfur proteins, which mediate electron transfer in a wide range of metabolic reactions. Several intriguing observations suggest that Fds may also directly associate with RNA, thus implicating a second role for these proteins in organellar RNA metabolism. Plants contain several closely-related Fd homologs, whose members are predicted to reside within the plastids. As strong mobile electron-carriers, able to partition between the stroma and the thylakoid membranes, Fds are therefore excellent candidates to regulate the expression of plastidic genes in a redox-dependant manner. Accordingly, the translation of D1 protein in the chloroplasts is mediated by a redox-poise involving the ferredoxin-thioredoxin system. Yet, despite these suggestive evidences, RNA binding activity has not been reported for an isolated Fd protein. Here, we established the intracellular locations of the six Fd paralogs in Arabidopsis to the plastids and demonstrated that one of these proteins, AtFd6, is associated with organellar transcripts in vivo. Biochemical analyses in vitro indicated that a re-combinant purified AtFd6-His protein binds with high affinity and specificity to psbA mRNA, in a redox-dependant manner.