Complementary DNA cloning and characterization of ferredoxin localized in bundle-sheath cells of maize leaves

In maize (Zea mays L.) two leaf-specific ferredoxin (Fd) isoproteins, Fd I and Fd II, are distributed differentially in mesophyll and bundle-sheath ce lls. A novel cDNA encoding the precursor of Fd II (pFD2) was isolated by he terologous hybridization using a cDNA for Fd I (pFD1) as a probe. The assig nment of the cDNAs to the Fds was verified by capillary liquid-chromatograp hy/electrospray ionization-mass spectrometry. RNA-blot analysis demonstrate d that transcripts for Fd I and Fd II accumulated specifically in mesophyll and bundle-sheath cells, respectively. The mature regions of pFD1 and pFD2 were expressed in Escherichia coli as functional Fds. Fd I and Fd II had s imilar redox potentials of -423 and -406 mV, respectively, but the K-m valu e of Fd-NADP(+) reductase for Fd II was about 3-fold larger than that for F d I. Asparagine at position 65 of Fd II is a unique residue compared with F d I and other Fds from various plants, which have aspartic acid or glutamic acid at the corresponding position as an electrostatic interaction site wi th Fd-NADP(+) reductase. Substitution of asparagine-65 with aspartic acid i ncreased the affinity of Fd II with Fd-NADP(+) reductase to a level compara ble to that of Fd I. These structural and functional differences of Fd I an d Fd II may be related to their cell-specific expression in the leaves of a C-4 plant.