Plastid import and iron-sulfur cluster assembly of photosynthetic and nonphotosynthetic ferredoxin isoproteins in maize.

We have previously isolated and characterized two cDNAs of maize (Zea mays) ferredoxin (Fd) isoproteins, which are differentially expressed in photosynthetic and nonphotosynthetic organs, and in response to illumination of the developing seedlings (Hase T, Kimata Y, Yonekura K, Matsumura T, Sakakibara H [1991] Plant Physiol 96: 77-83). To clarify the biosynthetic pathway of Fds present in the different organs, precursors of the two Fd isoproteins synthesized by in vitro transcription and translation were examined to determine whether they are imported to chloroplasts and etioplasts. Precursors for both Fd I and Fd III, a leaf-specific, photosynthetic isoprotein and a constitutive, nonphotosynthetic one, respectively, were imported into chloroplasts and processed to the mature size. Some of the mature-sized molecules inside the organelles were found to be assembled with an iron-sulfur cluster. The cluster assembly occurred without tight coupling to the translocation and processing steps of the protein import, and the process was time and temperature dependent and did not require light. Etioplasts were also capable of importing the precursor of Fd III and assembling the cluster. These combined data show that the constitutive, nonphotosynthetic Fd has the ability to become localized in plastids as a functional molecule.