Expression of ferredoxin gene family and its physiological significance

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Maize (Zea mays L.) has two types of ferredoxin (Fd) differentially expressed in photosynthetic and non-photosynthetic organs. To understand structural characteristics of the isoproteins and molecular mechanism of the differential expression of their genes, we have cloned two different maize cDNAs, which encode the entire precursor polypeptides of Fd I and Fd III composed of 150 and 152 amino acid residues, respectively. The gene transcript for Fd I was restricted to leaves and their levels increased markedly upon illumination, whereas that for Fd III was detected in all organs and its accumulation was not light dependent.To clarify the biosynsthetic pathway of the Fds present in different organs, in vitro synthesized precursors were examined to be imported into chloroplasts and etioplasts. Both Fds were processed to the mature size and assembled with the Fe-S cluster to become a functional molecule. The cluster assembly occured without tight coupling to the translocation, and the process was time and temperature dependent and did not require light.A cDNA fragment encoding the mature polypeptide of Fd III was expressed in E. coli, and the Fd was synthesized as a holo-form assembled with the cluster. Comparison of electron transfer activity of the two types of Fd showed that Fd III was superior as an electron acceptor from NADPH, and Fd I was superior as an electron donor for NADP, in reactions catalyzed by Fd-NADP reductase from maize leaf. The CD spectra of the two Fds also indicated a subtle difference in the geometry of their iron-sulfur clusters. These results are consistent with the view that photosynthetic and non-photosynthetic Fds may be functionally differentiated.