A cDNA Clone Encoding a Ferredoxin-NADP+ Reductase from Chlamydomonas reinhardtii

FNR (EC 1.18.1.2) is a flavoenzyme that plays an important role in the metabolism of photosynthetic organisms. FNR catalyzes the final step of the linear photosynthetic electron transfer chain by mediating the passage of electrons from reduced Fd to NADP+. FNR is situated at a branch point in electron flow, playing a key role in regulating the relative amounts of cyclic and noncyclic electron flow to meet the demands of the plant for ATP and reducing power. Besides these photosynthetic functions, recently FNR has been implicated in the defense response mechanisms against oxidative stress in Escherichia coli (Liochev et al., 1994). We report here the isolation of a cDNA clone encoding an FNR of the unicellular green alga Chlamydomonas reinhardtii (Table I). This clone was obtained as a false-positive signal from a cDNA library (Merchant and Bogorad, 1987) by antiphosphoglycerate kinase (EC 2.7.2.3) antibodies (Kitayama and Togasaki, 1992). The size of the cDNA insert of Chlamydomonas FNR is 1565 bp. The DNA sequence of Chlamydomonas FNR has 63, 57, and 58% homology with rice (Aoki and Ida, 1994), pea (Newman and Gray, 1988), and spinach FNR (Jansen et al., 1988), respectively. The deduced amino acid sequence of Chlamydomonas FNR has 59, 46, and 45% identity with rice, pea, and spinach FNR, respectively. Studies on the three-dimensional x-ray structure of FNR have focused on spinach FNR and the functions of amino acid residues have been assigned by Karplus et al. (1991). A11 of the amino acid residues that have been assigned specific functions, such as flavin adenine dinucleotide-binding residues and NADP-binding residues, have been well conserved in the FNR of Chlamydomonas. The homology within the putative N-terminal amino acid regions are very low when compared with N-terminal amino acid regions of a11 other species of FNRs. In addition, there is no significant identity of the putative transit peptide regions to the organellar FNR from spinach, pea, rice, and Cyanophora paradoxa (Jakowitsch et al., 1993).