Abstract
Ferredoxin-NADP+-oxidoreductase (FNR) is an enzyme catalysing the final step of linear electron transfer reducing NADP+ to NADPH. In Arabidopsis thaliana, the chloroplast targeted FNR enzyme exists as two isoforms, AtLFNR1 and AtLFNR2, encoded by two distinct nuclear genes. To reveal their functional specificity the knockout mutants of both isoforms were characterized. Absence of either one of the isoforms resulted in reduced size of the rosette with pale green leaves, which was accompanied by a low chlorophyll and LHC protein content, whereas the accumulation of Lhc transcripts was up-regulated. Knock-out of one FNR isoform also resulted in impaired carbon fixation. In the absence of AtLFNR1, AtLFNR2 was found exclusively in the stroma, suggesting that AtLFNR1 is required for membrane attachment of FNR. Structural modeling supports the formation of AtLFNR1-AtLFNR2 heterodimer that would mediate the membrane attachment of AtLFNR2. Dimer formation, in turn, might regulate the distribution of electrons between the cyclic and linear electron transfer pathways according to environmental cues.
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